North Iceland
The highlight of our time in the North was taking a whale-watching trip from Húsavík. This part of the North has the Diamond Circle. We didn't do the whole circle but saw most of the sights from it including Dettifoss and Goðafoss, as well as the geothermal areas. Having been to Yellowstone multiple times, the geothermal features were not as novel to me. The waterfalls are worth seeing though at this point in the trip, I had a degree of waterfall fatigue.
The major sight we skipped on Diamond Circle was Ásbyrgi Canyon. The pictures I'd seen made it look good but not a must-see. On the last day of the trip, we wandered into a photo gallery in Reykjavík with a photo of Ásbyrgi Canyon in the fall when the birch tree had fall colors and it was stunning so fall seems the the best time for the canyon.