this is what one of my friends did for her 18th bday party: she basically just had ppl come over to her house, just casually invited a lot of ppl. we had food outside and then a bonfire. later we rented some scary movies and only half of the group actually watched them, the other half was trying to scare the ones watching the movies. then the girls slept over and the guys had to leave at 1 am. hope i helped!