Even though it's free, you get two for the price of one! It's a bit of a detour from the 1 road (about 40-55km, depending on the route you take), but if you've got the time: go for it! From the 1 road turn on the 50, and follow it until you can get on either the 518 or 523 and then the 518..
The closest to the parking is Hraunfossar which is a very long stretch of waterfalls creating a cool wall of waterfalls.
A few minute walk to the east you'll find Barnafossar, a strong aqua-colored waterfall.