Watch wiring the audio. Do not cut any wires associated with the head unit, or you can trash some expensive stuff. You can probably upgrade speakers. If you have an amp that accepts high input levels, I know for sure you can add a sub amp. I ran one for a while with no trouble.

You could do cosmetic stuff. Get some wheels and tires. I can't imagine a spoiler kit would be a problem, unless you create some rust issues, but I'm not an expert, nor have I heard of a situation. You need a stiffer trunk spring to compensate for the added weight to the trunk as well.

Beware of lowering your car excessively. They could make trouble over that, especially if you have a body kit in conjunction to lowering. Techs get twitchy about them since they could be damaged by their lifts.

I'd suggest that you research any mod you plan on doing to make sure there will be no issues with reliability, and hopefully there will be nothing that requires warranty, but that's the gamble. Personally, I'm waiting a while before doing suspension mods beyond just the sway bar I switched out (which I recommend highly)