One thing my kids LOVE to do is "paint rocks" off the beach. It's so funny b/c you would think this is some BIG activity each year we go! My mom lays out a tablecloth with all kinds of paints and brushes. The kids search the beach for those perfect rocks to paint!
My mom takes one painted rock from all the kids to put in her landscape (big deal for the kids). It's so easy...you could do this at home. Of course, what is nice about painting at the cottage is when the kids are all done. Just tell them to run in the lake to wash off..now that is nice!