1) Arvada-Rifle (I70).map This is the road most travelled in Colorado by outsiders. Good thing it's so damn scenic. You'll be going uphill half the time to the summit near Vail and then downhill for the next hour or so. Weekend traffic will kill much of the fun.
2) Mt. Evans rd (off of 70). map pic1 If you've got the cojones to do it, this is a ride you won't soon forget. 20 miles from the bottom to the 14,000 foot top, there's no road more challenging nor more breathtaking. The last 2,000 feet or so it's gravel and near-constant hairpin turns. Along the same lines should be mentioned Pike's Peak road near Colorado Springs.
3) Estes Park-Idaho Springs (Rocky Mtn. NP). map pic2 pic3 High-mountain driving along the same lines as the Mt. Evans experience but with less demanding driving. Coming from Estes Park you'll soon be cruising a very well maintained and straightish road with gorgeous views to the 12,000 ft. summit then quickly descend the curvy mountainside to a lakeside valley some miles away. Passing towards the Winter Park ski resort you'll start going up again in an even curvier ride to the Berthoud Pass summit (11,000 ft.) and then down towards I70. April/May would be the best times imo.
4) Evergreen-Crested Butte (285,50,135) ~180 mi. map pic4 Running roughly parallel to I70 this is a great ride for those who'd rather steer clear of the mountain driving but like to soak in the views of the mountains themselves. Not much traffic adds a considerable degree of joy! Also, don't miss the meadowy area from Fairplay towards Canon City (285 to 9). map This is another top-notch drive.