Extras you don't need when renting a car.

There are several add-ons that car rental companies will attempt to get you to agree to.
Insurance; there is no need to take their insurance if you already have your own. Prepay for gas; this is a personal decision but the car rental company typically charges a bit more than a gas station. If you are familiar with the area and know of a station close to the airport, I would opt not to prepay for gas.
Satellite radio; unless you are going to be spending hours in the car this is just another way for the car rental company to charge you more for.