I have a few ideas; hopefully one or more will appeal.

1. Teifoc makes a lovely reusable brick and mortar building set. It might be of interest if they're enjoy construction:

http://www.eitechusa.com/Teifoc-School-Brick-Construction-Set-720-Pc-502.htm

2. Another idea might be family stargazing-- you could be thrifty and just use the Internet as a guide, or you could splurge and buy a telescope, field guide, etc.

3. I'd check out a cooking series on YouTube on the science of cooking. You could make your own composite foods using transglutaminase ("meat glue") like noodles made of meat, use liquid nitrogen to make ice cream, study the Maillard reaction, make pearls with basic tools like algenate, etc. Working on basic knife skills is practical, too. When DS19mo is old enough, we'll be turning our kitchen into a molecular gastronomy lab!!

http://www.seas.harvard.edu/cooking

4. Build your own computer from the ground up.

5. Write the script for and film a documentary on a topic of interest. If their friends are interested, they could do a short documentary film festival (ala Hot Docs).