My 21st is coming up, my dad knows the drill... a termignoni exhaust for the bike, and a good drinking session!
For my 18th I think I got an alpinestars leather jacket.
All very good suggestions if you have a motorbike, if not then maybe not so good.
I would just ask them to throw you the mother of all parties and get everyone you know and love in one place. If they still want a present after that then tell them to surprise you.