Knit caps are really popular right now, perhaps one that has a brim even? (I know I saw a pattern out there somewhere... just can't remember where...) You could possibly make one that had a soccer ball kind of pattern... but... that might a little too over the top even for a soccer loving teen ;o)
I would personally go with a beige/grey/brown/black knit hat. Something simple and ribbed.
EvanP on Ravelry (Hey that's rhymes!)