Denver's biggest advantage as a rental base is how quickly the mountains start. Within an hour of downtown, you're already climbing into serious elevation.
Mount Evans Scenic Byway is the highest paved road in North America and a straightforward day trip, with views that make the drive itself the destination.
Rocky Mountain National Park's Trail Ridge Road is another strong option — alpine tundra above 12,000 feet, accessible without any hiking required.
For something quieter, the drive along the Peak to Peak Highway between Estes Park and Nederland is less crowded and just as scenic, especially in early fall.