When you drive a car, you are inside of it, so you probably want it to be nice and safe. Roll cages are a great way to stiffen your card chassis, and keep you alot safer. Racing harnesses can also be attached to roll cages. Racing seats and harnesses are a great way to keep you from flying around the inside of your car while racing, they are also safer if you are in a wreck.

Car audio is also a popular thing to do to modify the inside of your car. I dont know much about this topic so I would check out some websites like NASIOC or I-Club. They both have Audio forums.

This is an example of a modified interior with racing seats.