Body-building...

Charles Bronson wrote a book about doing exactly that. It's called Solitary Fitness. I have a copy, unfortunately I don't have the necessary will power to do what it tells me to do.

Use resistance bands. Inexpensive, take up little room and portable. Plenty of instructional videos on Youtube.

Its amazing what can be done in a prison system locked up for many hours

For a dude your (our ) age it’s damn near impossible without HGH, but creatine does help .