DH's aunt and her dd love The Little Gym, not exactly sure when they started though. Could be fun, I think you'd have to look at what kind of stuff they offer in the class to know if it's right for your LO.
I used to do a bunch of things that fell in the middle of dd1's nap time when she was smaller -- it actually would work out fine -- generally I would put her down a little early knowing she would take a short nap and wake up in time to go to our class -- she would usually just take a longer nap later in the day to compensate for the earlier shorter one. Part of that is also your LO's personality - some can go with slightly altered naps like that, others don't do well with it at all. I think I just messed up her naps twice a week too, every day would be a bit much (not that you're planning on doing that). I would give it a shot and see what happens.