How about some bungee cords and a couple of eyebolts welded to the interior of the craft? It may not be Gold’s Gym, but the resistance should keep you from turning into a gersh on the trip. You can work arms, legs, back, even abs to some degree. Add more (or stiffer) cords to increase the resistance.
You can even call them Space Bungees and charge NASA $100,000 apiece for them.
Has to have a cutesy name before NASA will buy it.
Some cutesy anagram, like the Deepspace Unified Preventive Exercise Module or “DUPEM”