Travel from Siem Reap (Cambodia) to Bangkok (Thailand) could take you the whole day (by bus/train). I'm wondering if there is any sightseeing between Siem Reap and Bangkok where I can stop, sleep one night and then continue my travel. The idea is split the trip in two parts, but staying in a place where there is something to see/do.
There is no convenient midpoint stop, I think it's better to just bear the long road journey (I hear it's better in 2012 than few years ago).
Chachoengsao has a huge buddhist temple (allegedly the largest in the world), an old market and a decent river tour. Worth a visit, technically on the way, but rather close to Bangkok (1-2h).
Poipet is a seedy border town which may appeal if you're into casinos and gambling (and other activities which go along with that). Most people are happy to leave it behind after finding a bus or taxi onward.
Sisophon is tiny, and while you could probably find lodging there in a bind, doesn't seem too interesting.
There are plenty of places in Thailand that are nice enough to stay for a day. You'll always have access to good food and life, in general, is pretty relaxed. However, the only sight, as far as I know, that's really of interest and that's roughly between the two, is Ayutthaya. But that's so close to Bangkok, it sort of defeats your purpose.
at Pattaya, there is van to Rong Klua market. you must go early morning and rush to catch the van to across border and find out camry to Siam Reap.
Chonburi, the closed beach is Bang Saen and you can find the bus to Rong Klua with rush.
Prachinburi , you can visit by train but there is only medical museum and medical park at Aphai Phubet hospital.