We are doing this in September - flying to Bangkok and taking a side trip to Angkor Wat. You don't need a tour unless you want one. Have a look at http://www.tripadvisor.com/ShowForum..._Province.html . Also canby publications http://www.canbypublications.com/siemreap/srhome.htm .
Be aware that only one airlines flys Bangkok to Siem Reap - Bangkok airways for a ridiculous amount.
We are going for 3 nights. I can't see the point of going there for less than 2 full days. Lots of people seem to go for longer.
Otherwise if not Angkor Wat, consider some of the world heritage sites in Thailand. I joined a tour from Bangkok to Chiang Mai that visited ruins and temples among other things. It was some years ago and I really enjoyed it.