Answer: When it's the Rocky Mountain Arsenal National Wildlife Refuge!
Just outside Denver is a wildlife refuge that was once a chemical weapons manufacturing site in WWII and through the Cold War, and was later transitioned into a wildlife refuge in the post-Cold War haste to decommission weapons and also to clean up the environment. Now it's 15,000 acre wildlife preservation area with a visitor's center and interpretive trails, fishing, a photo blind, and lots of cool animals and plants everywhere. Swords into plowshares indeed.
We loved this place! The visitor's center was small but so great. They had some exhibits chronicling the history of the facility, complete with hazmat suits and homesteading costumes for kids to try on, dioramas, a gift shop and exploration backpacks visitors can check out (I'm a sucker for that kind of stuff).
There was also a fully stocked classroom which was open to the public when not in use for actual school groups, etc. This was a pleasant surprise, as generally rooms like this are locked or carefully supervised by volunteer staff. They had a reading area, some games and blocks, tables set up with activities and coloring, an impressive library and education guides for teachers. My kids loved it and would have stayed for hours if I had let them.
But we couldn't stay inside on a gorgeous Sunday afternoon in the middle of a nature preserve. So we hit the trail and went for a walk by the lake. We saw tons of cool wildlife: deer (including a mother with two fawns), prairie dogs (which really do pop up and say "yip" when you startle them), birds birds and more birds, and bison. Yep, bison. This trip is so cool and we're only 3 days in. We many never come home.
Everyone in our family gives this two thumbs (and maybe even an extra toe for good measure) up. Highly recommended!