Explore Seljalandsfoss and Gljúfrabúi Waterfalls

You can see Seljalandsfoss from the Ring Road (if you are coming from Reykjavik) so it's impossible to miss. It'll be on your left, and all you need to do is make one left turn on Rt. 249 (Þórsmerkurvegur) and you'll see a parking area (with a small bathroom).

You can walk all around Seljalandsfoss, including around the back of it. This is a very powerful waterfall so you will get wet as you get closer to it (and you'll definitely get wet if you go behind it). If you are trying to photograph it, bring a waterproof housing or a ziploc bag to keep your camera dry. You probably also want to wear some rain boots, rain pants, and a rain jacket so you don't get soaked.

Once you've finished admiring Seljalandsfoss, you will see the path continues to the left. if you follow this path for about 10 minutes, you will come to a small river that leads into a crevice on your right. You should hear a roaring sound coming from within the crevice. That's Gljúfrabúi! Leave anything you do not want to get wet behind (unless you have a very reliable waterproof housing for your camera, I would NOT bring it back here) and walk on the rocks through the crevice to the waterfall. You will 100% get soaked back here, so definitely wear all of your rain gear. There's a huge rock that people climb on top of to take pictures. Be careful not to fall off, as this waterfall is very powerful.