If you are planning to return the car early, be sure to check that you will not be billed an early return fee or have your rate quote change to a higher price per day. This is particularly true if you have rented a package and are returning the car before the end of the package time.
Avoid, whenever possible, using the speedy drop off. Take the time to arrive enough in advance of your flight to walk around the car with the attendant and sign off on the final inspection and bill. This can save problems with claims that damage was done to the car prior to the drop.