I haven't read all the posts, but have seen some creative ideas, and thought I'd add a couple things DC have enjoyed.
When DSs were younger, I would occasionally buy a box of individual aluminum foil sheets at a big box store, and the boys would be in heaven for days. They made everything you could possibly think of with foil and/or masking tape. The only problems were 1) that there were foil "inventions" all over the house, 2)they didn't want to get rid of any of them, and 3) sometimes they wrapped things up in foil (like little toys or money) so I felt the need to unwrap everything before I disposed of it. One of the best gifts DSs ever received was a wooden masking tape holder that holds 8'ish rolls, and each is a different color. It's still something at ages 7 and 9 that gets used at least weekly.
DS7 also got a lap-loom for Christmas and really enjoys using it when he has some time.
