13th Birthday Party Ideas?

My Birthday is In April, but my parents say we should organise it early, incase we have to book anything, but i am really stuck on ideas to do for my party! I would like to invites some friends to a REAL spa, not one at my house and then have a sleepover afterwards, but i can`t find any spa`s that will be *cheap* around £20 - £30 per head, and let me have up to 6 people, including myself. I dont really want to have a spa party at my house because i find they can sometimes get a bit awkward/boring. I dont mind if we can get someone round to do manicures/massages ect. Though. Please help me. Thank You :) x