Pro tip: Dollar Store has cheap supplies for backyard science experiments
Just did a volcano experiment with my 7yo and 4yo using stuff from Dollar Tree - got food coloring, vinegar, baking soda, all for like $5. They loved it, way less messy than I expected, and honestly way cheaper than those science kit things. We did it three times because they kept asking. If you've got little kids and want to kill an afternoon, highly recommend.
Just grabbed supplies for like $4.50. Gonna try this tomorrow with my 6yo. If this keeps him entertained for even 30 mins I'm calling it a success