Since Island Girl took my first thought, I'll add dirt. Let your kids play outside in the dirt. I had so many adventures building, 'excavating', and discovering when I was a kid in my own front yard! Played with cars in the dirt (I'm a girl, BTW) and made many mud pies! Considering I'm not even 30 yet, I suppose they can't be that old fashioned! Of course, then again my kid makes me feel old. We have sand in our back yard and the kids like to bury seashells (we live in AR, you gotta bring your own shells!) and see who can find them. Even I am finding them after a while! They forget where they bury them, but man they have so much fun and it doesn't cost a penny! Of course, you could always bury them for the kids and let them find them! A pirate treasure hunt perhaps??? Good times!