My sisters 18th birthday?

its my sisters 18th birthday in june, and i want to throw her an amazing birthday party. im younger than her. im 13. so me and my family are in this together haha! we want to keep this at home. what do we do? i wanna see if i can pull a special surprise off. the surprise is having my cousins come down from another state as a surprise. but i dont know if they can. they are really busy (they are famous pianists). so i need to talk to them about that first. i dont know what an 18 year old girl would want. knowing her, shes into the movie cars, the band mcr, umm im sure theres more important ones im missing. she even bought herself a DRESS for the special occasion. all she told me was to throw her an absolutely amazing birthday and pretend she doesnt know im throwing it. so what kind of stuff should i do? she has an xbox live 360, so mabey games?