Rocky Flats National Wildlife Refuge Trail is a 5.9 mile out and back trail located near Broomfield, Colorado that offers the chance to see wildlife and is good for all skill levels. The trail is primarily used for hiking and is accessible year-round.
I checked out this area visiting family here, and I was thoroughly impressed. There were plenty of people using the system, running and biking (I was running), but besides the volume of people the views were spectacular and the trails were very well maintained. It was a great surprise that something like this existed, and i'll be back using it when i'm in the area again.