I saw this online about the Beach Club but I don't know if it is up-to-date info. If it is true of other resorts, I know AKL (close to All Stars) has a nice fitness center:
"Beach Club resort guests (age 14 and older) can use the fitness center at no additional charge for the duration of their stay. A parent or legal guardian must accompany children under 17. Guests of other Disney resorts may do so for $12 per day, based on availability. The purchase of a spa treatment includes use of the adjacent fitness center for the day of treatment."