Skogafoss is probably the most impressive waterfall of Iceland. It's 60m high and 20m wide with lots of water. It's a very easy accessible waterfall with parking lot just 200m away.

This is a very popular stop so expect crowds.

If you can make it, head up the endless stairway, all the way to the top. This is the beginning of the Fimmvörðuháls trail - a 22 km trail that takes you between glaciers and into Þórsmörk. You don't need to do the entire trail though. If your itinerary allows, spend some time walking along "waterfall way" before turning back around.