(1)Stand outside in a pair of shorts (shirt optional. Being a girl, I like to keep mine on) and get someone to spray you with the hose for a while. Goggle also may be worn, just for the sake of being fancy.
(2) Water balloon/gun fights. Buckets are really helpful aswell
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(3) icepacks?
(4) Borrow your next door neighbour's kiddie pool thing. And their kiddie slide. COMBINE THE STOLEN GOODS.