I'm thinking of renting a car, as we'll be heading out to Red Rocks.
If you're a Costco member, you can get some great deals through them at
http://www.costcotravel.com Some are weekend & 3 day packages, some weekly rentals, free days, free upgrades etc. Lots of rental companies to choose from too, Alamo, Avis, Budget & Enterprise.
The deal I'm looking at is with Alamo, Denver Airport, Friday midday to Monday midday, midsize car, $81 including taxes.
Used Costco last year, when I travelled with U2Brad & Cathal from LA to Phoenix, Vegas, & back again. Got a very, very good deal on an Avis weekly rental. I'm not even a costco member, Avis never asked.