Taking the CDT in the Collegiates is a good one.
Two other options
from my little doc
Hope Pass: The old Colorado Trail route (and current CDT route) is harder but much more scenic than official CT. At 12540’ the views are breathtaking. Hope Pass is also the literal highpoint of the Leadville 100 ultramarathon that takes place in mid-August. After coming down Hope Pass, you go right and eventually connect back up to the CT on a dirt road. Or you can do the alternate below…
Missouri Gulch: Not too long after Hope Pass, you will go by the historic town of Vicksburg. This town is actually a historic site that has been re-built. Shortly after this town , you will come to a trailhead for Missouri Gulch. This alternate is far prettier than dirt road of the official CT. Much harder, though. This alternate has you surrounded by
three 14ers (Missouri, Belford, Oxford) and is an incredible view. If you are into peak bagging, this route is esp. good as the 14ers are easily accessible. The views from up to and at Elkhorn Pass are stunning. You follow the trail and connect back to the CT in a valley.