Walk around a ~3400 year old volcanic crater just off the Ring Road.
From the parking area just off the Ring Road (Highway 1) near the town of Bifröst, the trail starts to the left.
It is a very short, steep hike up to the top of the crater, but there are stairs and boardwalks to aid the ascent. At the top, there are wonderful views of the surrounding area and you can walk around the entire rim.
Exploring the crater takes about 15-20 minutes, so you could also explore the nearby Glanni Waterfall for some more exercise.