Yellowstone National Park is an amazing place : it has mountains, rivers, waterfalls, geysers and other thermal features, and lots of wildlife - black and grizzly bears, wolves, coyotes, foxes, buffalo, moose, big-horn sheep, mountain goats, a variety of smaller mammals, and lots of birds. There are 2 prime wildlife viewing areas, Lamarr and Hayden Valleys and lots of hiking trails. Bear spray is a must.
The biggest problem is the distances involved - it took us an hour to get to Hayden Valley from where we were staying in West Yellowstone. For early morning viewing in the summer, well, we woke up around 5 AM at the latest. It is all worth it, especially when you are fortunate, as we were. We got to see and photograph (mostly at full zoom) all the animals mentioned above, except the elusive moose, which we were assured we would see in Grand Teton NP (we did!).