Perhaps our all-time favorite state park (so far) is Custer State Park in the Black Hills of South Dakota.

A map:

Tunnel:

We had lunch at Sylvan Lake, then did an impromptu hike around the lake:

Sylvan Lake:

GIF of a hidden waterfall behind the lake:

Another tunnel:

Bison Center:

Wildlife Station Visitor Center:

Bison:

Pronghorn:

Prairie dog:

Visitor Center:

“Bison Poo” snack:

The Needles Eye:

Cathedral Spires Trail hike:

Legion Lake Trail hike:

A most excellent park.