The easiest things to do would be:
1. Intake Silencer Removal (doesn't take long and is free).
This will make the throttle slightly more responsive and make a cool intakey noise.
2. Get a K&N Panel filter. This provides a freer flowing intake and may give your car just a bit more power - only do this if you're not going to get/make a Cold Air Intake. This would be about $40.
3. Get One of Bob's (Anonda Motorsports) Front Strut Tower Braces. This will generally help the handling of the car - you can really feel a difference in turns. I love mine!
This is about $150 depending on if you get in a group buy or what the current price may be.
4. Get a custom programmed chip for your car that can firm up shifts and tune your car for a higher octane fuel and get a little more power out of it. These usually run about $300.
5. Custom Exhaust - a full cat-back exhaust will get you some HP gains and will vary and cost depending on what you get. I think, generally speaking, they can be done for around $400 (but I'm not 100% sure). I just switched to a flowmaster muffer, which helps with fuel economy, has a slight performance gain (mostly responsiveness I think) and sounds really cool.
Those are some of the basic mods you can do.