18 is just sort of like "whelp, I'm an adult now? Well I don't feel any different." 18 doesn't put an end to childhood fun, it, if anything, opens you up to the possibility of more freedom and going and doing as you please.
After I turned 18 I went and traveled for several months (took a year off of school) in Latin America. Would have been hard if I were still 17!
You are aware that nothing is going to change when you turn eighteen, right? Depending on where you live, you'll have a few more things you can legally do, but there isn't things you must do before you turn eighteen. You probably won't even notice that you are "grown up" until you're in your twenties anyway, so why worry?
And what is all that with making childhood count? Really, you're a teen, haven't you already left what would be called childhood, and what difference will it make whatever you do these next four month?
Do crazy shit, but don't do it because you're becoming an adult, do it because you like it and because it's fun.
Yeah, 'cause once you are 18, you can never go to con's again, or dress up in costumes, or watch anime. Geeze, I am more than twice your age, and have done all of those things well past 18. I would stop freaking, the day after your 18th birthday, you are the same person that you were the day prior to your 18th birthday.
simply be yourself, maybe they say that when you are 18 you must have a bf/gf and have drinked and had sex or whatever, but it's only crap, everybody has a story that differs, that is unique. I was totally the opposite at your age, I wanted so desperately to become adult, I wanted to have responsibility, take care of myself, show that I can make it on my own. 17 was probably the worst year for me, the last months before I turned 18, I hated the fact that my parents were controlling me all the time. When I became 18 I felt much more freedom, had my own money, made my own decisions, didn't need my parent's approval. I know many think that nothing changes when you become 18, but for me everything changed, maybe it depends on the circumstances. As others already stated, becoming 18 doesn't necessarily mean you are adult, it will certainly take more time than that. Turning 18 means you can decide on your own, but I know people that are 26 years old or even older, and still are so damn immature, behave like kids. And even as an adult, sometimes you will want to go back to childish things, nothing wrong with that.
I'm Nth-ing what everybody else has said; believe me, you're not suddenly a real adult at eighteen. I have met eighteen year olds who are mature, responsible, lazy, selfish, spoilt, childish, whiny, entitled, attention-seeking, kind, generous - you name it. They aren't anymore like adults on their eighteenth birthday than they were the day before.
Still, if you want to make the time count, what would you like to do? There's no point me telling you to go out, get wasted, get laid, get a tattoo, move city, question your sexuality and all that if you don't want to. So think of some things you'd like to try and make them happen, nevermind what anyone else says.
Personally I considered my childhood over at 13, so turning 18 wasn't exactly a huge transition. And you don't have to do those things you mentioned, like going to parties and getting drunk, simply because you're of legal age.