K.P.
I think this sounds great! You have a group of teenaged girls, food, music, and a pool... you don't really need to provide activities! You could do a photo booth (of sorts) with dress-up clothes, props, signs, etc and let them take pictures. You could then print them out and give each one a framed picture of the bday girl and the guest as a "favor". You would also have instant scrapbook material!