ShotHotspot has mapped 17 abandoned places in Boulder, Colorado, ranked by how often they're photographed, viewed and voted for. The most popular are Rusty Old Engine, Train Engine Colorado Abandoned and Sugar Mill Self Storage. Some are private property or unsafe, so get permission before you visit.
We don't know whether you're allowed to visit any of these places. Many are private property, closed to the public or unsafe: get permission from the owner or the relevant authority, and take safety precautions.