Depends on what you are looking for, performance wise. Suspension and brake/tire mods have the greatest effect on the car. Since it was pretty softly sprung from the factory, any suspension upgrade will have an immediate effect.
Engine mods - limited there, depending on how much you want to invest in this. Typical bolt-ons will improve throttle response and engine sound, but generally do little in terms of performance. Have to turn to forced induction projects or nitrous to get bigger gains. If tuned correctly, the 1ZZ-FE has a pretty decent power potential in stock form. To get more power, you'll have to pump in dollars to build up the engine. Head over to
www.monkeywrenchracing.com to see what is available. One of the few places that cater to the 1ZZ-FE and 2ZZ-GE engine families.