I'm on one right now (one month in)
If you don't have something planned, like going away, then the next thing you want to really do on a gap year is try to raise money for university. Either a full time job or a part time job. I suppose you also need to kind of delve yourself into new things if you're not going away because most of your friends will be gone.
Failing this (I partially fail - can't afford to go somewhere, but do have a part time job) set yourself a challenge for that year. Something you want to be better at. Personally, I aspire to improve on playing my instrument so that when I do go off to university, I can start getting into a lot of gigging. Your challenge can be different. I have goals and visions in mind of where I want to be by September next year.
If you don't plan on doing any of the above, or don't think you'll have the motivation to organize any of the above, I would highly recommend not taking a gap year. It will get very boring.
EDIT: With regard to being unsure. Yes a gap year can help. I'm still unsure but have that extra year to figure things out.