Begin at US Hwy 20 at Rexburg, Idaho, and stay on US 20 all the way to West Yellowstone, MT. There take Montana State Highway 191 to Grayling, MT, and turn west onto US 287. After viewing Hebgen and Earthquake (Quake) Lakes, continue on US 287 to the intersection with US 20, then turn south and head back through Island Park, ID.
This ride has something for everyone. As you ride north on US 20 you'll pass multiple farming towns and extensive agricultural and lava fields. On a clear day you'll be treated to distant views of the impressive Grand Teton mountain range.
Once you pass Ashton, ID, you'll cross the Snake River and start your climb into the flat-bottomed floor of the extinct Island Park super volcano. As you leave the vast fields behind you, you'll enter national forest and enjoy some sweeping curves and turns. The run through Island Park is largely a series of long straightaways through forests and fields. You eventually climb out of the other end of the caldera, ride through mountain passes and cross into Montana, arriving at the tourist town of West Yellowstone. From here, the ride is largely flat though beautiful as you travel north. Before long, you'll swing west and ride right along the shores of Hebgen Lake and Earthquake Lake. After you leave this incredibly scenic canyon and emerge back into the open, you'll turn south back through Island Park after you pass Henry's Lake off to your right.
The roads are generally well maintained. It's mostly a two-lane road, with a few stretches of 4-lane. This is the main artery between SE Idaho and Montana (west of I-15) so don't be surprised to see some large trucks on the road, but all in all the traffic is light.
There is no end to the number of scenic views, including mountains, forests, rivers and lakes. There are also large numbers of great places to eat in West Yellowstone (try Bubba's BBQ) and in Ashton, ID (El Rincon Restaurant). There is a nice visitor's center at the west end of Earthquake Lake as well as picnic tables and restrooms.