I'm 18 with not much cash and just bought a gorgeous 2007 yellow XTerra with a black bull-bar w/ aux lights, an airaid cold air intake, Kenwood aftermarket stereo already wired for a sub, some pretty sick 17 inch rims with normal sized all seasons on them.
I want to make this car the ultimate road trip car but I don't have much money to put into it. Any suggestions to make it look/sound/perform better? the only thing I'm not willing to do is do anything that will give it worse gas mileage

Not much money usually = no significant improvements.

Sounds like it is outfitted nice as it is. I would put money toward maintaining the mechanical aspects of the truck. Might even improve you gas mileage that way.