Get as many people as you can together (preferably outdoors, in a park or public place), arm yourselves with water pistols and water bombs, and attack!

You could split into teams, or just have a free-for-all; it's impossible to tell who 'wins', so it doesn't really matter (well, unless you want to go ultra-professional and put food coloring or other safe dye in your water...).

Works best on warm, sunny days...