Pay it off and as Dweezel said get a CC to use for regular small purchases through the month.

A friend of mine while still in HS got a CC that he would use to buy gas with once a week, at the end of the month he would pay it off and once in a while he would let it slip for a month then pay it off. He ended up with a crazy high credit score doing this by the time he graduated, basically letting it run into the next month once ina while shows creditors that you are willing to pay interest which they like because it makes them more money.

Not sure it still works that way, I don't do credit for anything just sharing what I have seen work.