wow, thats a lot of people. im in the same mess at the moment, i have like 30 boys and girls coming to a double sweet sixteen party of my friend and mine... if you know anyone form a local band, they might not mind playing for you for free.
you could have a bon fire
you could get some fireworks like sprinklers and those things where when u pull the string they pop and confetti flies out
you could make a weird movie
you could watch a movie at ur house
you could decorate at teh party the cake u will have for dessert