If you could let us know the age range for the babies or children that would be helpful! Classes can be VERY different for a mommy and me with infants, with young toddlers, or with preschool age. I'll look back later to give suggestions. Thanks!
Added:
For that age, I think an art class would be great, but I don't know how that would work into a fitness center. A better choice would probably be a dance class covering different types of dace: jazz, ballet, animal movements, hip hop, etc. Yoga is also a good option as well as gymnastics, which can be combined. Even an aerobics class could be fun at this age!