Tubing has been mentioned. I'm sure there are cheaper alternatives, but many moons ago my parents threw me a tubing party at 7 springs. Back then it was somewhat reasonable, I don't know how much it would cost today but I know skiing there has gone up considerably. Anyway it sticks with me as one of my favorite childhood memories. Part of that might have been I didn't get a lot of birthday parties that weren't at home because my parents didn't have much money until I was a few years away from being an adult. Regardless I really enjoyed it and planned tubing for my nephew that's about the age your child last year and it seemed like he and his friends had a blast.