There are a lot of "cold air intake" options (they won't produce more power, the OEM intake is already cold air), they do make more noise, though.
There are a lot of cat back exhaust options. Ditto.
Do a search here for past conversations about this: there is an aftermarket engine ECU program that many have liked. Its main benefit is reprogramming of the auto transmission shift points, and some other things (I forgot the name, mine is MT).
There are some other similar ECU programming options available, too.
There is also a Pedal Commander that makes the throttle more responsive and some find it quite enough for their needs, and its cheap.
For real performance gain, a supercharger is awesome (about 30% increase), but costs $5~6k.