Does anyone know if there is a decent hotel gym that non guests can pay to use around the Udom Suk, On Nut or Prakahnong areas? I just called the Jasmine Hotel at Prakhanong Bts but the concept of a non guest paying to use their facilities was too much for them to fathom....

Just go with an air of confidence. No joke, as a foreigner Thais are loathe to approach you, even if you could be breaking rules, often simply because they don't want to have to use English. They won't even know you don't live/stay there, just walk into any condo/hotel with normal clothes and your gym bag, inquire as to where the gym is (dont ask any other questions, just "where is the gym") and off you go. Now if the gym requires some kind of key card to access it (90% dont - but often the elevators for nice places do - which of course you can get around by using the stairs or just asking the people in the lobby for help), you may be out of luck. If you are truly daring, just ask a staff to open it for you.

If you present yourself as if you are someone important and should have access to the gym, the staff will almost everytime not bother you. Why? Because the thought of you scamming them out of their gym facilities is just too outlandish for them to grasp. Now, if you get caught by some brute who takes his work seriously, then you have no recourse but to fall back on the standard response of "im waiting for my friend who stays here"