Haha, this is so incredibly sad, but I truly can't remember what I did on my 18th! I mean, it was a long time ago, and it's not something I've thought about until I saw this thread, but I really have no idea!
I remember my 21st... mostly

But at least I remember what I did, but my 18? no idea! Probably got drunk and stoned.. then again, I do remember other things about when I was 17/18, so in actual fact, I probably just got drunker and stonederer! (I know that's not a word..).
Anyhoo, Happy Birthday OP, I hope you enjoy (and remember) whatever you decide to do!