17th Birthday Party

So I`m having a 17th birthday party and I am really excited! I got this amazing private lake for free ( I just have to mow the grass and clean up afterwards!) and I know we are going to grill and things like that. There will be about 40 high school juniors and seniors there, but I`m not really sure what to do. The party will be on November 3, at night, but I`m just not sure what else to do. Ideas anyone! I think I need a theme or something fun.