When I was 18 I made an appointment to have my hair done and to get a facial, simply because I wanted to and I could because I was 18

I wouldn't suggest it for anyone though because BOTH of the people I had were RUDE!!! and i think a lot of it had to do with my age. Maybe they didn't think i'd tip well?!? I would have tip GREAT, but they were rdue so i didn't tip them as much.
other then that, I would suggest the shows at night. Those are fun. Hanging up on the deck... having a couple drinks... playing bingo, i spent some time just walking around and taking pictures... there is a lot to do on the cruise, you just have to research a little.
Plus maybe you'll find someone around the same age as you that you can do stuff with? Good luck!