I can't stop thinking about STEM/STEAM projects. I suspect the fact that I get TinkerLab and Kiwi Crate emails might help. Both sites sell kits, printables, etc., but their activity pages are crammed full of stuff you can do without spending money.
Check out this do-at-home activity from Kiwi crate. The age range is 7 and up but doing it with littles - where YOU do most of the work and they watch - seems like a great idea. Look around at the Kiwi Crate page for other cool ideas. Zipline butterfly, anyone? (BTW, this on my Let's Make Stuff page for awhile.)
Do you want to set up a home laboratory for your young scientist/makers? TinkerLab has an article and a printable with tips to help you.
(In the case of bloggers like TinkerLab, I suggest clicking on an ad once in a while. Rachelle at TinkerLab offers so much great stuff - for FREE! Thanks.)