Travel!
Learn to drive
If you put off learning to drive during school time now is your opportunity to try. It will give you so much more freedom and independence, so book a theory test and once you have passed it the fun of driving really begins.
Get fit
Get a part time job
Learn to play an instrument
So you used to play piano? Lots of people who played for a year or two regret quitting- so get back on the horse.
Even if you didn't play an instrument you may have fantasised about being in a famous band at one point, so now is the time to make your wish come true. Guitar, piano, drums... the world is your oyster.