Yellowstone National Park was also one of my favorites of the whole US trip. At that point of our tour it was something completely different to see the geothermal activities of that place.
Unfortunately, the weather was too bad (snowfall in the middle of September) to make any serious hiking there, so we basically drove through the park and did some short hikes to several sights.

As soon as you enter Yellowstone National Park, you'll see quite a high number of bisons. These are the only free living bisons of North America. Although they used to be much more (60 millions in the middle of the 19th century) there are only 200000 animals left today, after they were diminished to a number of a few thousend in 1890, a couple of years after the park was founded. A bison is a really impressive sight and you can see them all over the park.

The major attraction of Yellowstone National Park is without any doubt the hot spring area around the geysir Old Faithful, who throws its boiling water up to 40 yards into the air and that regularely every hour (taken or given 15 minutes). There is a nice hike on walkboards through that area with steaming vents, geysirs and hot springs.

Another awesome sight is the gorge of the Yellowstone River, also called the Grand Canyon of the Yellowstone. It has a couple of waterfalls, the Lower Falls being the most impressive. You can get nice views of the gorge from the several viewpoints along the road, but also if you take the stairways to the Lower Falls.

Similar in beauty are the Mammoth Hot Springs in the north of Yellowstone National Park. In this place, the waters of the hot springs have built terraces of different colors and heights, which is a really beautiful sight.

Arches National Park / Bryce Canyon National Park / Canyonlands National Park / Capitol Reef National Park / Crater Lake National Park / Death Valley National Monument / Grand Canyon National Park / Lassen Volcanic National Park / Las Vegas / Lava Beds National Monument / Mesa Verde National Park / Monument Valley / Mount Rainier National Park / Olympic National Park / Petrified Forest National Park / Point Lobos State Park / San Francisco / Salt Lake City / Seattle / Sequoia National Park / Valley of Fire State Park / Yellowstone National Park / Yosemite National Park / Zion National Park