You know I honestly don't remember. I kind of stopped having big birthday parties around 14, aside from my 21st birthday, which was still pretty low key compared to the stereotype. I'd imagine I had a nice dinner, followed by cake, ice cream, and a couple of presents, but an otherwise normal day.
Oh, and happy birthday, OP!
Edit: I should mention, unlike turning 21 (which allows you to do something you can enjoy at any time of the year, with or without anyone else and without the risk of it slowly killing you unless you go /way/ overboard), all turning 18 does is let you vote, smoke, join the army, and get married. I have no interest in smoking or joining the army, I had nobody to get married to, but I voted a couple of days later. See, I turned 18 in January of 2008, just a couple of days before the democratic primary in my state. I was able to vote immediately because our supervisor of elections does a good job helping people in situations like that pre-register, so they get their voter ID card right around their birthday.