PSP or DS with games if you have any use for mobile entertainment.
I assume you have a good amp for your guitar and pedal?
I'm kinda in the same situation. I got a new co-op job that pays well (I'll net about $450 a week which is a crap ton of money when you live at home with the parents, no bills etc etc) so I gotta find some things to buy! I was thinking a guitar and amp since the guitar I've been so used to using is actually my brothers so I can no longer use it (its also an Ibanez, RG320QS or something). Or when I turn 21 in 5 months I was thinking of getting a pistol and a membership to a gun club just so I can get good with it. I also could use a major comp overhaul like a Opty, NF4, and a 7800GT but some days I don't even find my games that much fun so why spend so much $$ on something that is only for games. So I may just save for a while.
So in your situation I would prolly save it towards that car you're gonna get later. If you had say an extra $1k then you could get a nicer car than just from your parents. Or use the money towards something for your car like a cd mp3 player, etc..