The simplest activities are the best with kids! Make a spider web out of painter's tape, let them throw scrunched up newspaper ball to see if they can get them to stick. - - My kids can BE Spider-Man!
Make a bowl of popcorn and challenge your kids to play the Popcorn Olympics of straw blow.
Let kids practice their letters in this sugar writing tray. I'd probably use something else other than sugar, but still a cool idea.
50 Outdoor Summer Activities For Kids - Six Sisters Stuff