Decide when you will be traveling to Denver, Colorado or will require the use of a rental vehicle while in the area. Make travel arrangements for all flights, trains and hotels as necessary. Make sure to bring along your driver's license, a copy of your personal automobile insurance and a credit card, which is required for renting a vehicle.
Set a budget for how much money you have available to spend on renting a 4x4 vehicle while in Denver. Keep in mind the cost of gas that will be required for use in the vehicle, and also that the vehicle will need to be returned to the rental agency with a full tank.
Check with your personal car insurance agency to verify that your policy covers rental vehicles. Specifically tell your insurance agent that you will be renting a 4x4 vehicle and driving in and around the Denver area.
Decide which type of 4x4 vehicle you want to rent. Go to local car dealers in your home area and test drive any vehicles that you are unfamiliar with. Do this to make sure you will be comfortable with that vehicle's feel and handling.
Update maps on a GPS or other navigational device. Do this to make sure you are aware of where primary roads are located in and around Denver in reference to the off-road areas you plan to travel on. Map out areas you want to drive through and get directions to and from where you will be staying while in Denver.
Reserve your 4x4 vehicle through the rental agency you selected. Check the Denver International Airport website for a listing of rental car agencies that have a base of operations directly at the airport if you will be flying into the area.