OMG, could we have a more open ended question? What gun? The best modification you can make to any firearm is to break it in by shooting it. While not "cool" looking, it insures the gun is working properly and greatly improves your skill. IMHO adding a bunch of do-dads on a new gun is like buying a new car and having a new paint job done, new wheels and tires, chroming out the engine, replacing the shocks/struts, installing a new "booming" stereo, tinting the windows, adding a hood scoop, transmission shift kit, and redoing the upholstery BEFORE YOU EVEN TAKE IT FOR A DRIVE!!!!
It is a car, drive it. It is a gun, shoot it. Looks don't mean sh!t if you do not know how it works and cannot hit the broad side of a barn with it.
When you have a minimum of 500 rounds through it, you may re-pose the question.