Boulder is always in the top handful of bike friendly US cities in annual surveys (see, Bouldercolorado.gov). We have visited Boulder 3 times to cycle and have always been struck by how cycling-centric it is. Other than Rock Creek Road in the Owens Valley, we have not seen so many and diverse bike-related roadway signs anywhere in the US.
Top 10 list of hardest bike climbs in the Boulder area are: 1. Left Hand Canyon - Ward - Brainard Lake, 2. Magnolia Road, 3. Lefthand Canyon to Ward, 4. Sugarloaf Rd , 5. Flagstaff Road, 6. Left Hand Canyon (Jamestown), 7. Lee Hill - Deer Trail, 8. Left Hand Canyon - Super James, 9. Boulder Canyon, and 10. Sunshine Canyon
If Boulder is the hub for your Colorado cycling trip, you are in striking distance of the Denver Area Big Three: Pikes Peak (102 miles), Mount Evans 44 miles) and Trial Ridge (36 miles).