Welcome to HAN
Tires... the best damn bang-for-the-buck upgrade you can do... I'm not kidding... Get some nice tires that are suited for your situation (Like local weather and life/performance considerations) and you will be more happy than anything with nice tires...
I needed a lot of wet weather confidence with my car, with good dry performance and long longevity in mind, so I got myself some Dunlop Sport 5000A... what kind of needs do you have in your car?