In the United States, one is eligible to vote and is generally considered an adult upon reaching 18. Oddly enough, we don’t let you drink alcohol until you’re 21, but that’s another thread. If you can vote at 18 down under, perhaps you could print up a bunch of fake ballots for something? Have an election party? I have NO idea of what Australian politics are like. But hey, you SAID different.
I don’t know the laws in Australia, but if there are things that are now legal for you to do that weren’t at age 17, invite your friends to partake in all the stuff you can now do. Voting doesn’t have a lot of cache, but in the US, 18 means you can vote, buy cigarettes, buy pornography, get into a lot more clubs (but not all, thanks to the drinking age), hire strippers, get your own credit card… etc. Everything an adult can do, except drink or rent a car (or run for President).
So, depending on what becomes legal at 18 in Australia, pick your favorite vices from the list and go indulge them. In a rented car, if possible.