I REALLY DO NEED HELP!!! IT`S FOR MY SISTER!!!

Ok my sister is turning 14 years old on July 21st (2006) (duh) and so since she doesn`t like to throw parties of her own (because she doesn`t want them to be boring and stupid) I decided to throw her a surprise party.. I`m hoping that it turns out great but i`m afriad about if it doesn`t... I need ideas on what for her friends to do at the party.. But like i can`t have dumb silly lame games cuz the kids are ranging from ages 15-18 (not including my sister obviously) and they are all pretty mature besides if i give stupid games my sister won`t like the party.. Can anyone help me? With anything at all? Planning? Great ideas? Any ideas? oh by the way i have a pool and it`s all outdoors (i also have a little house outside, it`s our workout room)