2.5 days in REP is enough time for most people to see the temples of Angkor. I would probably take an early flight to BKK and head to Chatuchak (Weekend) Market where you could easily fill your suitcase with last minute souvenirs.

If you have never been to BKK, I think you will appreciate the extra day in BKK. I'm biased however...I love BKK, but there are many out there who don't like the city at all.