This one comes via my Incredibly Creative Sister:

For her son’s birthday parties she would give each kid a garbage bag with openings cut in for the head and arms, give them all shower caps, swim goggles, and a can of shaving cream.  Then just set them loose.  She gave them each a carton of eggs the first year too — That didn’t work out quite so nicely…

The parties were so successful, Big Sis used this theme five years in a row. My Nephew D and his friends are now all in their late teens and still talk about these parties.