Some of my favorites from this lise: build a fort, nature walk, scavenger hunt, cook/bake, make and blow bubbles, tye die, homemade salt clay, make a milk carton boat, pet rocks, "travel" around the world (food, games, etc. from all over), make slime... Good Ideas, Kids Stuff, For Kids, Free Summer, Summer Activities, Fun Ideas, Fun Things, Great Ideas, Summer Ideas
Also on these boards
33 Activities Under $10 That Will Keep Your Kids Busy All Summer - there are some *awesome* ideas here!!! :)
We did a much lamer version of this... only on paper, on the fridge... it's a good idea though!