Some minor, or at least relatively minor things you can do are switching out the header, as has been suggested, and switching your intake manifold. Maybe get a slightly larger throttle body. These are all relatively small upgrades you can do in less than an hour... and upgrades that don't require any more fuel. Your ecm can compensate for them in stock trim. It's not tons of power, but if thats what you're looking for, you're going to invest a lot of time and money. You might also look at suspension mods. They don't make your car necessarily faster, but they do make it more fun to drive... and a lot of people take that aspect for granted when they start modding. You can have as much fun going fast slowly, as you can going fast quickly.