Hike to Red Mountain in Northern Arizona.
The Flagstaff area has several dormant volcano cones scattered around. If you're interested in seeing what the inside of one looks like, then you will enjoy this easy walk.
If you're taking this hike in the summer, you'll start from the parking area. If it's winter, the short approach road may be closed, in which case you'll add 0.3 miles to your hike. Either way, you'll follow an old road towards the easily visible Red Mountain. The trail is easy to follow for it's length. When you reach a rock dam (with a set of stairs) you will have reached the inside of the Volcano.
This area looks like a set from a Star Trek movie, with bright red walls, rounded spires, and numerous holes. It's worth spending some time to explore around, but be cautious in climbing, as the rock here is crumbly.
When you've had your fill of the bizarre scene, return the way you came.