Only about an hour's drive outside Reykjavik following Route 1 (the Ring Road) is one of Iceland's best waterfalls. The country is ribboned with them because of all the glaciers that melt and cover the island, but Seljalandsfoss is particularly awesome for it's size and accessibility.
You can spy it off the Ring Road to your left from fairly far away; once you get closer you'll see a bend where you can turn in and park. The falls are magnificently big and meet the earth at velvety green land; on a sunny day, a rainbow might form.
If you're there in the summer, take the stroll behind the falls in a rain jacket (it's misty) and listen to the cascade from the natural enclave. On your way out, notice that the rumble and echo of the water from behind the falls sounds like planes soaring overhead. Delightfully tricked, I tucked that moment away as a reminder of how refreshing and provocative remote nature can be.