Grinnell Glacier

This hike is within Glacier National Park, and is incredible. The views along the trail are almost better than the actual destination, hiking alongside and overlooking 4 mountain lakes: Swiftcurrent, Lake Josephine, Grinnell, and finally arriving at Upper Grinnell to view Salamander Glacier. The trail is considered heavily trafficked, and is pretty exposed the entire distance so can become hot and dusty in later summer. It is just over 11 miles roundtrip (the distances can vary because there are multiple places to access this trail), and has about 2100' elevation gain.
It's best to arrive at the trailhead early in the day as the parking lot fills up really quickly.
The trail winds over a footbridge, then along Swiftcurrent Lake, crosses a forested area to arrive at Lake Josephine, and from there starts to climb. You gain elevation pretty quickly, and are soon looking over Grinnell Lake with Angle Wing peak as a backdrop. The last section of the hike has some steep drop offs to navigate and switchbacks up sharply before arriving at the lake. Bighorn sheep frequent the area.
The Many Glacier entrance of the park is not currently requiring a ticketed reservation, but the road has been under construction the last several summers, so check the status and possibly expect delays. Glacier entrance fees are $35 for a week, or $70 for an annual pass.