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I agree with Kevin on this, spending some time at a place gives a deeper experience than making it a day-trip. Manop is a great option based on the age of your son. I can't comment on the advantages of gyms based on their locations and proximity to tourist attractions as that's not something we do when we visit gyms, but Chiang Mai in general is easy to navigate and has lots of things to explore. Bangkok is harder to move around in, but also lots to do it you take half a day or a day off.