I'm surprised no one suggested a 2 hour traditional thai massage especially after a long flight!
Try "Health Land", a chain of massage parlours who offer authentic massages i.e. full body masaage, foot massage, etc at a very reasonable price. I usually go for the 2 hours traditional thai massage, and it is so relaxing that
ALL the time I will fall asleep during the course of the massage. ^
Go to the Chong Nonsi branch. Take the BTS (skytrain) and alight at the Chong Nonsi station. Then walk towards the building that looks like a ROBOT. And yes, it does look like one from far. The ROBOT building is the UOB Bank Building, and the Health Land branch is directly opposite the ROBOT building.
It can be very crowded during the evenings so I would suggest you make a prior reservation. It closes at 12 midnight, thus the latest you can book the massage (to fully enjoy it) is at 10pm.
Enjoy your stay in BKK.