Here is a activity I did with Adam, Austin and Grace this past summer.  Baking bread.  I bake about once a week and if Grace is here she needs to help me.  Glen now is wanting to play with some too now.  When the grands are here they all get some dough to make their own creation and get to take it home.  I am sorry I didn't let my children be creative, but I really didn't bake bread then.  But I have to add, they all are very good at baking now.
Being Grandparents is a lot different than raising your own.  I'm thankful, that now at 70 I still have enough energy to keep Grace and Glen while Lana subs.  Also, keep the others as needed.