We spent a long time in Thailand, enjoyed the beaches (partied a little too hard maybe) and the north but I would avoid spending too much time/money in Bangkok. A day for the temples is probably enough. Khao San Rd is the seediest place I've ever been EVER but it's fun and easy to meet friendly travelers.

Cambodia is nice, Siem Reap is a MUST, we spent a day at Angkor Wat and it is one of the most serene places I've ever been.