We have been to Yellowstone almost 20 times, most recently a couple of weeks ago. Do you want to concentrate on the thermal features (the largest number in the world) or concentrate on animals? The thermal features are on the western side of the park while the best animal sightings are in the east. A lot also depends on when you are coming. The hotter the weather, the harder some of the animals are to see. For examples, most bears and elk are up in the high country by late June and so harder to see. We were there this year for a week in mid-May and saw 9 bears (black and grizzly), numerous wolves, bisons having their calves, elks (who hadn't dropped calves yet), deer, mountain goats, mountain sheep, etc. We generally stay in the West Yellowstone area, but it is a long way to the Lamar Valley where the best wildlife viewing is. Are you flying or driving? Have you made lodging/camping reservations? The park is crazy crowded this year, and it is hard to find a place to stay.
There is a great app called "Just Ahead" that is an audio tour guide. It knows where you are and tells you about the location. Well worth the money it costs! And I would also say a bus tour is a good idea if you have limited time. Of course, you will certainly decide after your first trip that you have to return!
Best of luck.
Pam