A Real Miniature Car, Powered by Hydrogen Fuel!
Up for a little time travel? Let's way go back to days of morse-code and the early "wireless". I haven't tried these kits yet, but they look very interesting: Radio Receiver and Morse Code Kit
The product description says that they're best for ages 10 and up. However, they also say, with adult supervision, they're good for ages 3 and up. Cool! Sounds like a family activity to me!
Shape Scapes Sculpture Set pieces have curvey shapes. Not your average construction set. While supposedly a kid's toy, it's also a nice desk toy.
Kaleidoscopes are cool. Building your own is very cool.
Authentic Models Seeing Stars Kaleidoscope Kit kaleidoscope kit
I haven't tried this kit, but it looks pretty straightforward and easy to build...with styrofoam balls as phospate groups and sugars.
FloraCraft Fun Learning DNA Kit DNA model kit
This is a really fun building set for little hands! It's hard to find fun toys for the under-three set, since most interesting toys aren't rated as safe because of choking hazards.
Lego Mindstorms! This is at the very top of my 7-year-old's wish list. To be honest, I can't wait to play with it too!