Depends on your play style really, personally i would go with a good red dot sight first of all. Internally, a better spring and a tightbore barrel would help out quite a bit with range (especially if you are playing outdoors). I would get metal bushings installed and have the mechbox re-shimmed, (but leave this to a gun doc if you've never opened a gun or don't have someone to teach you).
Other than that, tac-lights, lasers, slings and all that other tacticool goodness. Oh and maybe a get a digital charger, wall chargers are shit,