Hmm.. checking connections this might be an option, and adds some desperately needed relaxing moment:
1 arrival
2-5 Bangkok and surroundings
6 all day train ride to Chiang May or 'something else'
7-10 Chiang May or 'something else'
10 evening-13 Siem Reap (plane)
13 evening -17 beach not too far from border Cambodia (bus or private driver)
18 take public transport, flight or private driver back to Bangkok, fly home in evening