Wired.com has two great ideas for toys/projects to do with your kids.  I haven’t tried them, but they look like a lot of fun!

First, build a trebuchet.  A whut?  A trebuchet.  It’s a medieval-era projectile launcher.  Okay, so this is an outdoor toy.  And probably best for kids 8 and over.  There are no instructions here, but there’s a good picture and funny story about the misadventures (and the learning) they had while building it (Courtesy of Adam Savage–of MythBusters).

Second, how about a personal hovercraft?  For real.  Instructions are far more specific in this one–although be forewarned that it requires a leaf blower.  The article says that an electric one will do–but I have an electric, and I can almost guarantee that it requires a beefy gas-powered one.  And make it removable–you might want it back.  The picture includes a joystick, but you’ll have to figure that one out on your own.  But just imagine the fun of building it with your child, and the fun you’ll both have pushing them around the yard.  Until your back gives out 🙂