Anytime Fitness is rapidly expanding in Okinawa.
Considering it's a 24-hour gym, you get a lot of bang for your buck. The one that I went to wasn't packed at times either and it was like my own private gym. The machines and equipment there are nice, new, and clean. I took a few photos of the inside one that is in Southern Naha pictured here
Here's a map of all Anytime Fitness franchises on Okinawa:
One negative that you might not like is that they do have a Covid-19 mask and a dress code policy (no showing tattoos).
Speaking of dress codes, I saw FOGO Gym
in Chatan being advertised to me on other social media platforms stating that they have none (probably within reason). This might be a better option if you're unable to get in at Anytime Fitness due to a language barrier; though, I don't think that'll be an issue once you give them your credit card to automatically make payments.
If you don't want to commit to monthly fees, you might want to look into your local city gym that would charge something like 200 yen per visit.
It would help me more if I knew where you were located so I could narrow something down for you within a 15-minute warm-up jog.
Hope this helped!