Other ridiculously fun ideas:
take a shaving cream bath
paint your entire body
buy water soakers and get in a squirt gun fight
do nothing but bake all day, filling your kitchen and table with plated goodies, and then invite your friends over, your kids friends, your family
every family member in a costume, all day, everywhere you go. no explanations
dye everyone in your family's hair the same temporary color
(then take a picture!)
go on a mud-hike in the rain (we've done this, so fun!)
clear out your living room and have a dance contest
!