fitness class with 7 year old son

I'm trying to work out what sort of class I can take with my 7 year old son, so we can learn something together. I think we'd both get a lot out of it. He'd see me learning and trying hard, I'd have to always be a good role model (it would keep me honest).

We go swimming together for fun, and he plays tennis each week, I go to a ukulele class but I want to do something active where he can see me learning but where I'm not going to be so far ahead of him that he can't relate.

I'm thinking maybe a martial arts class of some sort. Has anyone got any experience of taking classes with their kids and if so which ones were best?