Kerið is a pretty volcanic crater lake located in the Golden Circle area. I recommend stopping here in the summer when the colors of the water, soil, and foliage really pop! Kerið is not a must-see destination along the Golden Circle route as it is somewhat out of the way but if you're heading to the South Coast or back to Reykjavik, consider adding it to your itinerary.
Once parked you may walk around the rim of the crater and down into it next to the water's edge.
Parking is free but a small admission price per person is required.