cold air intake, if not cold air intake get short ram intake. performance spark plugs, new exhaust system (performance headers, high flow cat converter and muffler), and other simple stuff getting replace will also help. fuel filter, higher quallity oil and oil filter, fuel system cleaner, new plug wires, etc.
No matter what you buy, it won't make an appreciable difference in performance, unless you install a supercharger or turbo.
Anything you do to install something that you think is giving you more power will result in more fuel costs. You will tend to push the skinny pedal on the right down a lot quicker. Is it worth it at $3.50 a gallon?
If you want more power, sell it and buy one with a V8.