Cold Cathodes, and UV cable sleeving. Once you buy cold cathodes try to buy any remaining "trimming pieces" as UV reactive. Cabling, fans, connectors. It starts to get expensive, but buying CCFL's alone doesn't do much. They produce very little light.
EDIT: But for other mods:
-Laser cut fan grills.
-Flashy paintjob on any given part (doesn't always have to be the case).
-Another big one: Bay devices. They can range from fan controller to speakers. Hard drive chillers, RAM intakes, LCD readouts, etc...
-If you didn't have an Antec 1200, I would suggest a window mod. But I suppose you would windows the top panel. Sometimes that looks good. Depends on taste.
EDIT2: I'll give you a link to where I shop for crazy mod parts. They are a bit expensive but will definitely help you get ideas and then you shop it around and see who carries it cheaper.