Great views and interesting rocks, a brief geology lesson.
Panum Crater is a rhyolitic volcanic cone close to Mono Lake. Geologically, it is very young, just 600–700 years old. This trail heads up to plug at the top, crossing over pumice and obsidian. The initial 0.05 miles from the parking area doesn't look very promising but at the junction with the Panum Rim Trail yellow wildflowers and sage fill the landscape. At 0.25 miles, after a few switchbacks, the views of the Sierras, White Mountains, and Mono Lake are good and only get better. The second half of the trail is less distinct but still fairly easy to follow. While taking in the views, don't forget to look down at the interesting rocks.
The gravel road to Panum Crater from Highway 120 is marked. It is suitable for low clearance vehicles at a modest speed.