Yellowstone has over 10,000 geothermal features, many of them are geysers. Infact, it has the largest concentration of geothermal features on the planet, and it was this that was a key reason why the park was created.
Yellowstone has it's own Grand Canyon. While not as deep, or wide, or long as the actual Grand Canyon, it's just as beautiful. The range of colors is much more vast, with great contrast between the yellow of the walls and the green trees trying to grow there.
The Grand Prismatic Spring is one of the more spectacular things Yellowstone has to offer. If you can get high enough to see it that is. And even then you have to hope the wind is blowing in a favorable direction. There's just so much steam coming off it that takes patience to get a good view of it.
The North Area of Yellowstone is dominated by Mammoth Hot Springs. Due to the low elevation of the area, less than a mile above sea level, the Mammoth area is open year round. It's also home year round to a healthy herd of elk who are not afraid of humans. Just north is the town of Gardiner Montana, and the Roosevelt Arch.
Norris Geyser Basin is home to Steamboat Geyser, the world's tallest geyser. Unfortunately, it doesn't erupt quite as regularly as the other geysers of the park. Norris Geyser Basin was one of the first places we went in the park. The early morning was full of fog, and quite eerie, not to mention freezing. We went back another day in the afternoon and it was much more enjoyable.