15th Birthday Party Ideas?? Help..

Ok, I`m turning 15 in lke 12 days... Yeah i know im a procrastanator (sp?).. Anywho, I`m like so out of ideas.. I know that I wanna have at least twenty people.. I live near the beach, and the water parks, and the mall, and all that junk.. But ... Lets just say I`m not the richest person around, but I wanna have a fun and cool party.. If ya`ll could help that would be like so amazing.. Oh yeah, btw this party is gonna be only girls if that helps.. Thanks.. :D