This^
You could replace.... the motor. With an ls1, and you'll gain LOADS of horsepower.
If you don't want to swap, and you're not looking to make a ton of power, then your average bolt ons, like an intake, header, exhaust, etc. It will give you a little extra get up (not a whole lot) and give it a good sound as well. A common thing that many people ignore is how well your car runs as well. Do a complete tune-up, replace your fuel filter, plugs, wires, cap, rotor, and troubleshoot any current issues you have.
Other than that if you want to make some serious numbers you could always boost it. But there's a lot to learn before going that route.
and there's plenty of swaps available too.