Suspension mod:
- 99-00 Si rear sway bar (bolts right up, you can do it with a socket wrench)

Other cheap show suggestions:

- replace your headlight bulbs with Sylvania Silverstars. They're legal but give you that HID-ish whiter looker.
- Pep Boys/Autozone/Kragen/APC fog lights.
- muffler TIPS are dress ups only and do nothing
- straight through mufflers do practically nothing unless you do the header and cat back with it
- change your shift knob
- add wheelskins to your steering wheel for nicer grip (and it can be subtle color or wild color)
- add pedal covers