Racing strip (possibly the cheapest mod you can do and look good)
Alloys
Body kit or Replace skirts and spats
Front splitter
Spark plug cover
Neons inside car (outside ones are illigal when turned on)
Black and white number plate (yes there legal)
Spark plug cover
Coloured hoses
Sport exaust
Air filter (K&N are the best)
Ice (sub,amp,6x9)
Bucket seats (make car much lighter)
Straight through exaust (costly but increase performance)
Paint repray

You can never run out of things to modify on a car
Check out car forums to get ideas of what to do