There are quite a few attractions visitable as day trips out of Bangkok, most easily on tours that may combine two or three of them in one day .
West of Bangkok
Perhaps the most popular trip from Bangkok is the 109km journey southwest to the picturesque Damnoen Saduak Floating Market (ตลาดน้ำตำเนินสะดวก) in Ratchaburi province. This market is very popular with tourists, though it is still a little better than the one near Wat Sai in Thonburi. The main floating market on Khlong Damnoen Saduak (Damnoen Saduak canal) is Ton Khem market, but there are others at Hia Kui and Khun Phitak. Hia Kui is the most touristy and has boats mainly of souvenirs for tourists but others are not quite as bad. Really, it would be hard to justify any trip to the Floating Market if it wasn't for the fact that Bangkok is famous for them.
Most people come here on a tour but it really is best done independently if you want to avoid the crowds. It takes place in the morning and tours tend to arrive at 9.30am, so those who arrive by 9.00am or earlier will have the best experience. This also leaves you free to hire a boat yourself and look around the canals and markets, this costs about 300B an hour. To get there early, take one of the buses from Bangkok's southern bus terminal in Pin Klao - They start at 6.00am, go every 20 minutes or so, cost around 50B and will normally take about two hours. Alternatively, there are a couple of hotels in Damnoen Saduak itself.
Tours to Damnoen Saduak will very likely also visit the Rose Garden and the chedi at Nakhon Pathom. The Rose garden is set in large landscaped tropical gardens but is mainly visited for the 'cultural show' that's put on there - Thai traditional dance, Buddhist ordination ceremony, muay thai (Thai boxing), elephants at work, takraw (a common sport in Thailand) etc...It's totally touristy and as contrived as can be, but if you know what you're in for it can be reasonably entertaining nonetheless. Also in the grounds is a luxury riverside resort, thousands of rose bushes and hundreds of exotic birds. Close to Rose Garden are the Thai Human Imagery Museum and the Samphran Elephant Park. The human imagery museum is a Thai Madame Tussauds with images of famous monks, all the Chakri dynasty kings etc...This is really aimed at Thais rather than foreigners. The Elephant park has a variety of elephant and crocodile shows (wrestling, tug-of-war etc..)
Nakhon Pathom , around 60 km west of Bangkok, is a town famous as the reputedly the oldest in Thailand and the place where Buddhist missionaries first entered Thailand from India. Nowadays it is a fairly typical provincial Thai town with only the gold encrusted Phra Pathom Chedi hinting at its celebrated past. This is a massive 127m high, making it the word's largest Buddhist monument, and the most important in Thailand. As well as the main chedi, there are numerous bots, Buddha images and other structures nearby. There are hotels nearby, but this is mostly visited either on a tour from Bangkok or on route to Kanchanaburi.
South of Bangkok
South of Bangkok, in Samut Prakan province, are the Ancient City (Muang Boran) and the Crocodile Farm. The Ancient City is a 200 acre outdoor museum, roughly in the shape of Thailand, which contains replicas of the main attractions found throughout the country and some of the replicas here no longer exist in their original form. There are over 60 monuments in total appropriately located throughout the grounds, and the grounds themselves are landscaped with small lakes and waterfalls, rock gardens etc...It's best to come here on a tour, or with your own transport, as the grounds are too large to walk around. The website is at http://www.ancientcity.com/. The crocodile farm has a huge number of crocodiles (it claims 60,000+), with the feeding and crocodile wrestling show being the highlight of a visit here. The fate that ultimately awaits crocodiles here is their skins being turned into wallets and briefcases, and the meat going to nearby restaurants ! Somewhat ironically, it has been remarkably successful at preserving the crocodile in Thailand - they have just about been hunted to extinction in the wild. The crocodile farm also have a website at http://www.crocodilefarm.com/.
North and East of Bangkok
Another popular day trip is the journey on the river to Nonthaburi, a densely populated town north of Bangkok. (It's sometimes considered a Bangkok suburb, as Bangkok's rapid growth has meant it's hard to tell where one starts and the other finishes). There's little of interest in Nonthaburi itself, but it's often visited as the northernmost stop of the Chao Phraya river express boats. The return trip from the Oriental hotel pier, which lasts around 2.5 - 3 hours, costs only 20B (approx US$0.5) per person. Despite it's proximity to central Bangkok, it has a definite provincial feeling - be prepared for something of a culture shock if this is your first trip into rural Thailand. Attractions are limited to a few wats, the Singha brewery and the infamous 'Bangkok Hilton' - the none-too-pleasant Bang Kwang prison, where foreigners convicted of drug related crimes invariably end up.
To the northeast of Bangkok in Minburi suburb are Suwan Siam Water Park and the Safari World park. The water park is set in large landscaped gardens and has artificial surf, whirlpools, fountains, waterfalls, large water slides etc...Other attractions here are children's playgrounds, aviaries, an open zoo and botanical gardens. At 170 acres, Safari world is the largest wildlife park in Southeast Asia. This has a wildlife section, with 75 animal species, including animals such as giraffes, zebras, elephants, lions and tigers. The marine section has trained dolphin shows and an aviary section with eagles, macaws, parrots etc...There are various shows available (including dolphins, birds, seals, monkeys). This is best visited on a tour, as the wildlife section is for vehicles only (though the park also has coaches you can go through on, and most taxis drivers can be persuaded to drive through it if you offer them a bit more money). The other sections can be visited on foot. Their website is at http://www.safariworld.com/.
An hour north of Bangkok by bus is the former summer residence of the Ayuthayan kings at Bang Pa-In. It was abandoned after the move of the capital to Bangkok, but later restored by King Mongkut (Rama IV) and used occasionally by King Chulalongkorn (Rama V). It was abandoned again after Chulalongkorn's wife and children drowned in the nearby Menam river, despite being surrounded by loyal subjects. This was due to regulations governing the relationship between royalty and commoners at the time, stating that any person who touched the royal person to rescue them were to be executed. The complex itself is elegantly designed and reasonably interesting, but is only really worth visiting as part of a trip to Ayuthaya, 20km to the north. Bang Sai, the Royal Folk Arts and Crafts Center is also around the same distance away, and was established to try to keep alive traditional Thai technologies and life e.g. basketry, wood carving etc...It's only really worth a visit if you have a special interest in this subject.
A 160km, two hour trip, Phetburi (sometimes called Phetchaburi) is just about visitable in a day trip. It's worth a visit for the historical park with it's many old temples, spanning a time gap of some several centuries. It's one of Thailand's oldest towns, but was never destroyed by the Burmese unlike others and so much of the old temples are still intact. There's also the bright white Khao Wang, a palace built by King Mongkut (Rama IV) in the 1860's.
It's relatively easy to escape the chaos of Bangkok for more relaxed surroundings. They are all places that can be done comfortably in a one or two night trip from Bangkok. Some of them, such as Ayuthaya, are possible day trips, but all have enough to see to justify staying longer.
The former capital of Ayuthaya lies north of Bangkok, and take two or three hours by bus or train. It's also possible to go there on the river, though this takes around 4 hours. It's a large, impressive site with many ruined temples and buildings, and is perhaps the best easy excursion from Bangkok.Kanchanaburi
Kanchanaburi is about 3 hours west of Bangkok, and is most visited because it's the location of the 'Bridge Over The River Kwai', made infamous by the movie of the same name. Nearby are the beautiful Erawan waterfalls, widely regarded as Thailand's best.
The beach resort of Pattaya is about 2 hours from Bangkok by bus. The beach is second rate compared to almost any other in Thailand, and the water too polluted to swim in, but it's still very popular due to it's legendary nightlife scene. There's hundreds of bars and restaurants and thousands of prostitutes, all of which attract the 3 million tourists that visit every year. It's not a total tourist trap though, nor is it by any means as expensive as it might sound. It's also Thailand's best place for watersports.
4 hours south of Bangkok is Hua Hin, the more relaxed and somewhat upscale alternative to Pattaya. Hua Hin has a better beach (though still not great compared to those in the south), and is cheaper. Nightlife is virtually nonexistent. It's mainly visited by those who can't get down to better beaches in the south, such as Phuket or Ko Samui. Nearby Cha-am and Prachuab Khiri Khan are similar.Ko Samet
The beaches of Ko Samet are beautiful, easily rivaling any of the more famous southern islands. Parts of it are very touristy (as it is so near Bangkok), mainly Diamond beach which has upscale hotels, discos etc... mostly catering for wealthy Thai tourists. Pollution is a problem in this part of the island.
Other parts are much less developed, but generally food and accommodation are overpriced all over the island. Most of the accommodation fills up on Thai national holidays, weekends and during the tourism high season. It's around four hours from Bangkok to the pier for Ko Samet, plus the boat trip to the island itself. Ko Samet is a national park and charges foreigners 200B to enter the island, 10 times what it costs for Thai people.Khao Yai National Park
The large Khao Yai National Park is about 4 hours north east of Bangkok, just inside the northeast Isaan region. It's quite a good place for spotting animals in the wild, species include elephants, types of deer, bears, gibbons and there's even been sightings of tiger and leopard.Lopburi
Briefly the capital in the 17th century, there's still the interesting former palace and temple ruins to see in Lopburi, but nowadays it's most famous for the huge amount of monkeys that inhabit the old city.