You're flying BKK-HKG, are you originating in BKK or connecting from somewhere else?
The 30 minute massages aren't too bad at the spa, although I find the foot massage much better.
There are also a couple of spa/massage places out in the general concourse of the airport.
If you are connecting via BKK note that if you leave to go to Novotel you might have to pay the Thai departure tax on your return through. Small risk, but it has been known to happen.