Unsurprisingly there are a few fitness establishments in this Westernised backpacking mecca. But there's only one with a decent amount of weight lifting equipment. Located in the Buddy Lodge Hotel.
The gym is a litte more equipped than your standard hotel gym and is fairly quiet most of the day. There are dumbells, a bench, smith machine, dipping/pullup station, some resistance machines and treadmills.
Details as of 01/09/2016:
Cost: 200 BAHT per day
Dumbells: Up to 25kg
Barbell w/ plates: Approx. 140kg
Bench Press: Yes
Squat Rack: No
Entry Requirements: None as I could tell, flip flops were ok.