Definitely the highlight of Yellowstone in my opinion is the Grand Canyon of the Yellowstone River. It's almost unreal with its colors of yellow, green and red, but it really looks like this:
We took a hike on the north rim of the canyon and got some great views of the upper and lower falls. Amazing to see it up close:
The only problem is that we had to walk back up 600 feet from the brink of the lower falls. We drove around to the south rim and got these classic shots (same as all the postcards):
What is so impressive about Yellowstone is its size and diversity. Besides the canyon and waterfalls, there is also the unique thermal features like Old Faithful and the grand prismatic spring, a huge alpine lake, abundant wildlife and some great hiking. We spent a week here and saw different things every day.