Pilates can be very challenging, if you make it so. It is also very simple to start out... a good instructor and/or video will tell you how to simplify (or complicate

) each move. I personally can't stand yoga. I know a lot of people love it, but it is too boring for me and a little goofy. Sorry to anyone who likes it, Kyle does too.... so please do'nt take it personally

Both will help with flexibility, but I think Pilates, IMO, tones and increases flexibility more than yoga.
I have done Pilates for about 10 years now when I started with a particular ballet company. I have nothing but good things to say about it, and could go into more detail about what I like if you wanted (either here or via PM).
As for joining that rec center, I say GO FOR IT! I thoroughly enjoyed working out in high school and at Miami because of group exercise classes. Each type is so different, and there are so many things available to try! I had a blast. I wish I could say the same now

Good timing, though. I just went to a class today called "Cardio Step Challenge". I thought it sounded like fun, but I don't think I'm going to be able to breathe tomorrow!

Regular, basic step classes are very easy to learn though, again IMO. And my mom thinks the same thing, so I don't think it has anything to do with dance, etc, trust me
Another one I loved? Jazzercise

It's cheesy as all get out but I had so much fun! I wish my gym offered it, but it doesn't and I don't need to join a jazzercise center in addition to dance and our gym.
Hope those help! Like I said, I have lots of opinions on group fitness classes, so feel free to ask questions. And sorry if I answered too much!