Depends on what you want to do:
Some basic things for the engine are: Cam, Valve Springs, Port and polish (can do yourself with a Dremel), Adjustable cam gear (More practical with a modified cam a dyno), a remapped ECM
Basic suspension things to do are: Upgrade the Bushings (always fun - flaming marshmallows from hell), springs (cutting saves ride comfort), shocks, and Sway bars (the best - dont hurt tide quality and REALLY help response and handling)
Stay away from Cold air intakes - they dont help that much. A header (headers depending on model) can help, but be careful - they can reduce backpressure and HURT performance. a header is a good thing with a port and polish and pistons. Then a cold air intake would be a good idea.
Avoid underdrive pulleys - they will kill your alternator. They may look cool, but they really suck in terms of acessories.
Tires are good. Best upgrade for the money. Low pro are good - to a degree. Too low, and the arent any better than the size below. The best size are dependent, but 16-17 are the best. Also, light rims are a really good upgrade. if you can finda rim that is 9-10 pounds, that is increbily light and realy good.
If i can think of anything elses, I'll post more. But those are general enough.
Good... Bad... I'm the guy with the gun.