I was about to start a new thread on this very topic 2 ask if n e 1 knew of a cheap place you could use on a one off basis, most places wont let you in for a single workout.
I found a nice place off Soi 1, you can get there from Soi 3 by walking through Boomboomgrad Hospital. Its called Vital Life Wellness Center and costs 300 baht a workout
Lumpini Park now has a new machine and dumbbell gym which costs like 20 baht a YEAR but you need a passport copy and medical health certific8 and there are strict rules like not shorts or tanktops so I never bothered with it.
Also in Lumpini is an old outdoor gym been there for years, very cheap but the equipment is a bit rusty and dangerous