This was one of my favorite parts of my trip, the water really isn't that cold once you get use to it. I wanted to stay in even longer.
https://i.imgur.com/YJdZ8Sw.jpg?1
If you bring waterproof gear (you should) you can walk up to and hang out in these waterfalls.
https://goo.gl/maps/xedKvu3ifNB2
https://i.imgur.com/NsdlDUB.jpg
If you are going to Skogafoss I would suggest eating at the fish and chips stand, it was our cheapest mean at "only" $20. But it was also great, we went twice.
https://goo.gl/maps/9cNTjMZKxcH2
Gullfoss was amazing (
https://goo.gl/maps/aP3F3sn9tXA2) plus you can stop and eat at a farm along the way, one of the best meals we had.
https://goo.gl/maps/EeVs6qj2h9D2
https://i.imgur.com/K6ernb5.jpg?1
If you are heading out to the penensula I would also suggest eating at this little cliffside restaurant, its tiny, and you might have to sit outside in the cold, but the seafood soup is the best I've ever had.
https://goo.gl/maps/zoVzaM1pZP52
Obviously all the main attractions are great like the plane wreck, black sand beach and diamond beach ect. I would skip the blue lagoon and find a smaller more remote bath. We went here, much cheaper and less crowded.
https://goo.gl/maps/nvwKFyhucRR2
PS They have more sheep than people, so keep your eyes on the road!
https://i.imgur.com/StEWUWa.jpg