That's really a question of personal taste. What do you want out of the car? How do you drive? How much are you willing to spend? Those are the questions you should ask yourself. Any car is worth modifying if it will make it more enjoyable for you to drive.
There are a bunch of cheap mods you can do to liven up the car, and make it more visually appealing.
Personally, I would start with a K&N Filter, or a new intake, and a high-flow resonator or other exhaust modification. Performance-wise, those are easy modifications, and probably your best bang-for-the buck.
Do a search of the forums for exhaust and intake suggestions, there have been tons of them!
Good luck!