For my 6 yr old son this years party was a rock climbing theme.  We sent out invitations literally written out on rocks the size of your fist with pertinent information. We rented a rock wall that was set up in our driveway and they also had a bouncing castle for the children who were waiting their turn to climb. We had a slab cake iced grey with Oreo cookies standing up on it to simulate the foot/hand holds. Our treat bags were papier mache rocks that I spent a few days painting grey and writing each childs name on with black marker. Inside was home made rock candy, rockets, and other little treats.  I sealed the tops with grey duct tape and voila!! All the kids (16) had an excellent time climbing and bouncing! The party lasted two and a half hours.