There is a very nice spa up the street from the Ambassador: Ananda Spa. You should be able to google it. For a lower to mid-range priced spa, Raintree on Soi 11 (west side, near the Rosabieng Restaurant) is OK also. Both of these offer packages, both are also legit spas. Best to book at least a few hours in advance. I've heard good things about Urban Retreat also, but haven't tried it personally yet.

The commercial massage services at the Airport are overpriced and quality of staff not consistent, so best to have a real spa experience in town.