April Fool's Costume Party?

How about decor and activities commonly associated with a mish-mash of other holidays, i.e. Christmas ornament making, jack-o-lantern making with gourds, fruits, vegies, sparkling cider toast and old lang syne at midnight, valentine's hearts-maybe pin the arrow on cupid, etc...

Do a water balloon toss, they're lots of fun.

use innocent gag gifts like the fake fly in the ice cuhbe or whoopie cousions. There's also he gum that turns your mouth black etc... You can find most of this stuff at Spencers Gifts... :)