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!

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

How could I resist inflatable planets? I think I'll be ordering a set for my kids....who am I kidding, I want them in my office.
I'd probably not use the sun in this set, since the scale is so far off..the real sun is nowhere near the same size as any planets ;-)