Gluggafos (aka merkjárfoss) is a very easily accessible double waterfall that you can sneak behind!
Super simple, super easy to get to. Just a cool little waterfall located just off the road. Surrounded by nothing but farm lands, it gives you a great spot to pull off and enjoy a beverage or enjoy a bite to eat. The water is a double waterfall, and the lower half gives you an option of actually walking behind the waterfall.