collect rocks and paint them (my favorite)
paper mache a ballon and paint the "heads" to look like each other
yarn / needle crafts
clay and play dough
cooking
finger painting

Go on pinterest and do a few searches for kids crafts/ summer activities / outdoor fun and you'll be surpised at the creative ideas you find.

Take an old sheet and put it over a table and just let them paint it.

Origami it up. A book increasing in difficulty can keep them occupied for hours.

One word... Pinterest
They have an awesome recipe there for industrial strength bubbles. Not to mention lots of other ideas.

My parents put me to work before entertaining me. Do they have bikes? Outdoor activities will wear them out.

Canoe or kayaks, bikes, walk the dogs, kitchen help make dinner, cut coupons, give each skid half the grocery list and whoever saves most gets some of the savings for desert, or allowance, etc. Garden. Build a dog house, tree house, shelf, dresser. Firestarters with egg cartons, wax and dryer lint. Explore fire house, treasure map.