For years, I got free usage/membership at a very high end spa/gym in exchange for teaching a martial arts class.
A lot of Gyms offer free usage in exchange for services of any kind (teaching aerobics, daycare, ...etc). It's a good deal if you have the time to do it. It always ends up taking up more time than you think it's going to though - that's what makes it hard.
You just need to make sure that the gym you're working for has a good policy.