The Rocky Mountain Arsenal National Wildlife Refuge is a great place for wildlife watching just outside of Denver, and has been surprisingly uncrowded both times I have visited. You can watch cute prairie dogs, birds, the occasional deer, and bison! In some areas the bison are in large fields and there is a fence between the bison and your car.
And in other areas the bison are free to walk across the road and right up to your car! It’s very exciting, and at least a little bit scary, to see these giant animals (the largest land mammal in North America!) just a few feet away.*
*These photos were all taken on my phone without any zoom.