Are there any good group fitness activities?

I have been involved in fitness for the last 4 years. Recently I find that my gym program has gotten pretty stale and I'm looking to liven it up a bit. I'm looking for a group fitness activity that will improve my cardiovascular conditioning, help me trim fat and teach me self defense, without the formality of karate or judo (where you have to have a uniform and all that)...something fun...any ideas out there?