My oldest son wanted to do a backyard camp out with his friends for his 10th birthday.
I think if you have too many planned activities scheduled, the kids get bored. They just wanted to hang out and play. They played baseball, had squirt gun fights, ran around in the dark late at night chasing each other with toy guns and flashlights, laughed so hard telling each other goofy stories in the tent and they barely slept. Sounds like a blast to me!
Anyway, these are some of the things we did.