Ayutthaya

Ayutthaya is an ancient Siamese capital - of the Ayutthaya Kingdom - which was sacked by the Burmese in 1767, an event which still features strongly in the cultural memory of Thailand (and probably the reason why Burmese are portrayed as dark-skinned barbarians, even in contemporary media). The city was once on of the most important diplomatic and trading posts in the region, and the numerous ruins make for a fascinating trip out of Bangkok even if you are not a history buff. It is probably also the easiest day trip you can do (logistically speaking). The ancient city is located just 1.5 hours drive North of Bangkok, and there are tons of transportation options.

Tours are about 1000 baht for four sites, inclusive of entrance fees and lunch. However, since it is a small town with everything within easy reach, you can leisurely tour it at your own pace with a rental scooter (150 baht/day) or a tuc-tuc tour (3-500/day). For small groups with 2 persons per scooter and cheap meals, the whole day including transport and two meals can be done for 500 baht/person.

Suggested itinerary:

-Grab a mini van from Victory Monument (60 baht one way). The area is big and has many bus stops, so be sure to take exit 4 from the BTS and (after a quick u-turn look) it will be the first bus holding area on your left-hand side.

-The mini van will drop you off half a dozen kilometers from the old city. You can here either rent a tuc-tuc (be sure to negotiate, as they will try and scam you) or take a Songthaew bus to the railway station for 15 baht. From the railway station, you can get a light lunch and rent scooters ("motorbikes") for 150 baht/day (be sure to add 20-30 baht of fuel before returning it). Scooter rental places can provide you with free tourist maps.

PRO-TIP: It is highly recommended that your first stop is the Ayutthaya Tourist Center, which has a free exhibition that gives a great overview of the city's history and provides context about the temples and daily life of the region. This way you can easily plan which destinations you want to visit (many skip the beautiful Chai Watthanaram ruins because the map makes it look like its relatively far away, but its just on the other side of the Western river and takes barely 15 minutes to reach from the tourist center by scooter).

-See 3-4 sights and have dinner before dropping off your scooters back at the railway station. The last train to Bangkok departs around 7pm, and costs 20/60 baht for third/second class (the latter is recommended - upholstered > wooden seats). Instead of going all the way back to Hua Lamphong railway station, you can get off at one of the many stops inside the city including Victory Monument.