Indoor Rock Climbing
Yesterday I went to a rock gym after work with a guy from work. I've been there before and both times it was a great workout. By the end of the night my fingers and arms felt like jello. I sure felt the burn of that last climb. Some of the people there are in crazy good shape. They put me to shame something fierce, but I still had a lot of fun climbing.
Anyways I just thought I'd mention this since you can probably find an indoor climbing gym in your area and it's certainly worth trying if you've never done it before.
Have fun climbing.