Boulder Open Space The city of Boulder has a great web page devoted to accessible trails. There is also an amazing online book with detailed descriptions of Accessible Trails & Natural Sites in the Boulder area. The floods of 2013 severely damaged many of the trails. Check with Boulder Open Space for trail conditions.
Marshall Mesa Trail Boulder describes this trail as “appropriate for wheelchair users who are looking for an extra challenge”. I should say so. I watched a boy of about 10 who was having difficulty bicycling up the short rough stretch that is about 13% grade.
Walden Ponds is at 75th St. between Valmont and Jay Roads. The reclaimed gravel pits are a great place for viewing waterfowl. The 2.9 miles of flat dirt trails connect to the also very easy Sawhill Ponds trails. There is a wheelchair accessible boardwalk, a picnic table, and a fishing pier. When dry, most of the trails appear to be wheelchair accessible.