Not knowing what your look is now, I say go with some fun lip color and eye shadow. Or do you mean clothes? get somethign with a few sparkles, or some fun new jewelry, lots of bracelets or some fun new dangly earrings. is this what you mean?
I'd go for some highlights that match the tone of your hair. If you're planning on going to parties and such, I'd pick up some hot rollers and stick those in your hair. Pick up some decorative hair pins to add the final touch.