Ayutthaya north of Bangkok is an old city with a great Buddhism heritage. It is a history showcase of Thailand or as it was known before Siam. A daytour to Ayutthaya is probably the best when in Bangkok should inlude a visit to the historical park and a river cruise with a long tail boat (about Baht 500,- they start from several places in the old town).
One monastery (pictured below) is exposing interesting murals and wall painting depicting the war with Burma just see below. If you want to have some fun with nightlife take a motor cycle taxi to the new town at the grand place they plenty of bars entertainment and restaurants there. Ayutthaya is living Buddhism and what is still to see from pagodas monasteries art and other interesting stuff. The next best and somehow similar place in the country is Sukhothai.
Another tourism highlights the river and canals tours around the old city core which are built on an island in between three of them. The city is not particularly big and most interesting parts are on the island surrounded by three rivers and canals as visible on the old map below.
There is also a new part which is about ten minutes with the motorbike taxi and grouped around the grand square, that's the place to go in the evening. It also interesting to visit one of the restaurants floating over the rivers, that's in the old town.
In general when you book a hotel try the river resorts it' somehow nice to open the window and have the smaller ships plus the gigantic barges passing, actually there is an amazing busy traffic moving up and down north south. Sometimes the city is also named "Venice of the East" and indeed there are a couple of similarities but on a much lower scale and beauty.