Brakes, aftermarket pads will help a lot. I am getting the Porterfield R4S as I recall(Same thing as the Comptech Hot Street pads)

 

you say you dont want to do any mods b/c of your warrenty and then your say exuast are too loud and your tryin to grow out of this car thing? do you want to mod your car or not? suspension would be my first IMO.
You have a fairly fast car to begin with. if your worried about the warrenty do mods that you know you can switch out farily quick. what are you looking for engine upgrades, exterior/ interior? suspension? theres tons of options for you jus narrow them down by finding cheap, easy to uninstall performance parts u like!

theres the template now bust the piggy bank and pimp that thing out! have fun.

 

Dude get a header and or exhaust, it won't do anything to void the warranty, unless you get a header with no O2 or EGR fittings. What kind of Nissan did you have? $tillen sucks ass BTW.