Hyundai elantra Enhancements? Or Engine, Performance, Enhancements?

Basically i was given a car, and little by little i'm putting money into it, it has taken good car of me; so far i've put a new set of rim wheels on it and tinted windows and a new stereo. so now i wanna go a bit further and instead of appearance i wanna enhance the performance on it, my friend said that a cold air intake would help the engine but a bit more horse power, what else besides that would help my engine? or enhance the performance of my car in general? oh and i'm driving a 2003 hyundai elantra. anything would help, thank you very much, and money is not an issue.