sorry: anniversary was yesterday, I wasn't checking all my forums
There is a ballet school on the corner of Rt 1 & Main St - I think it's upstairs. don't know a thing about it, other than it exists. there may be another dance studio around that corner, not sure.
don't know a thing about gymnastics (I don't have any kids, so...) have you looked into the Boys & Girls Club on the corner of &th & Montgomery (used to be the old high school, still looks like it). I don't know what all they have, but I think they have some things.
Fairland complex is technically in Montgomery, but they kinda have a deal with PG as well. Yes, you can take classes, join the gym, go swimming (really nice set of pools - caters to the kids). You don't have to buy a membership, you can pay per session, or buy a block of sessions or longer - unless they've changed things in the few years we've not been going there.