Stomp rockets are also fun to do with your grandson! He was amazed that it flew!

Cool. When they get old enough make a particle detector.

You could also cut it up, being sure to leave an "eye" on each piece, and toss all the pieces into a 5-gallon bucket, filled with about 2/3 dirt.
You'll actually wind up with a bit of a crop.
Works with sweet potatoes, too.

I really agree.
I used to do the same with my sisters kids when they were young. She was a single parent, and I helped watch over them when she was at work.
I was never pushy with them, I did what was done for me. And that was to make Science fun.
And like you we got to do all kinds of fun experiments.

Years later, And I still get some 'thank you's' from them.
I'm just happy they got to experience critical thinking.