You'll pass by Georgetown, Colorado on I-70 west of Denver (I second Kevin's suggestion for route). Be sure to stop there and look around, it is a neat little town. There was a mining railroad there that operated over a circular trestle -- called the Georgetown Loop. It was an amazing feat of engineering for its time -- and is still spectacular today. They may still operate narrow gauge excursion trains on it -- if they do, take a ride!

Check all the major rental companies for your best deal -- the one that has the best price today may not be the best one tomorrow. The best advice I can offer you is to stay away from the second-tier companies -- stick with the majors, like Hertz, Alamo, National, Advantage, Enterprise, Budget, Avis, etc. You have less chance of getting ripped off. Pick up the car full of fuel, and return it full. The other "deals" they offer are no deal at all, when you look at total costs -- especially the one where they offer to sell you the tank of gas up front at a supposedly lower price and they tell you you can bring the car back "empty."

That's nonsense -- you will never take a car back completely empty -- it is bound to have three or four gallons left in the tank at least -- which is gas you paid for. Then they sell it to someone else at the same "great" price. It's a great deal for them, but not for YOU. Don't fall for it.