Do you want to drive normal cars like a Camry/Accord or do you want something like a Mercedes E class or an Audi A6? For normal cars Avis should also have the 2010 Taurus, which is fairly interesting. The fleet mix may even include an SHO, so walk the lot first to see if you can spot anything you like.
Business trips should be a chance for you to drive something you don't normally get to drive at home. If you've never driven an Escalade, Hummer, Camaro etc. now's the time to try them out. Avis also has a bunch of the new CTS/STS/SRX cadi's that are in their fleet. Remember to pick the loaded model, ie. the ones with the panoramic glass roof, dvd player, etc. Its all the same price...look outside and point to the car you want.
Just keep in mind, Enterprise/National/Alamo (same company) share the same fleet, as do Avis/Budget, so choose whatever is cheapest and upgrade to something nice.
*To OP, the Avis rate engine isn't that sophisticated, so you are probably doing something wrong. You could call them?
*Enterprise sucks.