You could still have a nice more formal than not party. Cook some nice food, or even get some pizzas ( the contrast with some dressier cloths would be kinda funny). Appatizers. music. Trival Pursuit, Pictionary, etc. (I don’t know if that is hip with your crowd, usually if you set the formal side people are open to it and then it gets fun). Maybe have set up a video game system if you have one. A movie if you have some. Just a nice gathering of the friends that mean a lot to you as you turn 16.