The USA Today has put together a list of the best urban bike paths in the USA. It look like both Boulder and Denver have made the list.
From the USA Today:
2. Boulder Creek Path—Boulder, Colo.
Rising from the plains to the Rockies and bisecting this mountain town from East to West, the paved, 7.5-mile Boulder Creek Path epitomizes what makes Boulder one of the most active, outdoorsy cities in the country.
5. Cherry Creek Bike Path—Denver
Denver's numerous paved bike paths extend citywide and beyond, earning the Mile High City annual props on Bicycling Magazine's bike-friendly cities list. The standout (among many) is the Cherry Creek Path, which extends from the confluence of Cherry Creek and the South Platte River in downtown, all the way to Cherry Creek Reservoir 12 miles to the southeast