What should i do for my 13th birthday it has to be really cheep or free and there will be like 18 ppl?

like 18 ppl boys and girls free or very cheep and something that is REALLY fun i have already had a plain party and a sleepover and all that stuff i dont like seeing movies and some of my friends cant watch pg 13 stuff also i dont like the mall and my house is SO SMALL like two bedroom duplex also i live in portland oregon so if anyone knows any fun local places here that would be cool my birthday is in like 9 days plz plz help!!!