You will have to The party can be at home or perhaps at a rec center rented for the day. Another idea for a party is to hire a DJ and have music. Give them tickets and let them spend the whole day there, then come back to your house for the pizza or ice cream or whatever. So another idea is to take the teen and friends to a theme park for the day. But perhaps your 18-year-old-to-be would find the above idea too childish, which it sort of is. With a build-it -yourself dessert bar with ice cream and all the fixings. An all-pizza party would be great.
An easier party to have at home can center around food. Or maybe have the party at home with a frat/sorority theme and for food feature “mystery meat”. Perhaps a visit to the college and a celebration at a nearby restaurant would be pleasant. The birthday celebrant is probably starting college soon, and so a college theme for the birthday might be nice.
One idea is to perhaps coincide the party with going to college. Here are a few 18th birthday party ideas that you might consider. So it would seem that a party to honor such an occasion should be fun, yet not seem too childish. It signifies a true beginning to being a grown-up.