Birthday Party Ideas For A 15 Year Old Girl!?

Okay So I Am Turning 15 Soon And For The Longest Time I 've Wanted A Fun Birthday Party But The Only Problem Is That I Want To Invite Guys And Girls And Both To Have Fun But I Can't Have The Party At My House Because I Live In Apartments And It'd Be Too Small To Have A Party In It. I'm Not The Richest Girl In The World And I Want To Save Money For My Mom. I Am Planing On Inviting Maybe 15 To 20 People And I Love To Have Fun. Any Ideas That Are Fun But Cheep. Thanks Guys!