I'm a big fan of the Family Fun site - they have so many cool ideas to keep kids entertained (plus they have an excellent app for the iPhone and iPad). Every project I've made has actually worked and the kids love it.
Yesterday we made Paper-Plate Polyhedrons.
Took us about 10 minutes and a .99 pack of paper plates.
I'm sure there's a math lesson in here somewhere about polyhedrons (I'm sure a homeschooling mom out there will jump right on this), but the finished product is cool enough on its own.