We traveled to Yellowstone during the height of tourist season but this did not deter us from seeing the many impressive features that the part has to offer. We also had our young children with us (ages 8 and 5) and this was a wonderful experience for them! Our first order of business was to visit the Albright Visitor Center at Mammoth Hot Springs, where we signed up the kids for the Junior Ranger program (worth it if you are traveling with kids) and where the WHS plaque is located. I suggest that you leave your car parked in the visitor center's parking lot and then walk the short distance to the Mammoth Hot Springs, which is an interesting thermal feature. There are also lots of elk roaming around the town, especially at dusk.
The diversity in animal life is fantastic. Head to the Lamar and/or Hayden Valley to see herds of bison and pronghorn. Twice we experienced a bison bull walking right beside our car. It's mating season so the bulls are on a mission and it's not uncommon to see them walking on the roads. We also saw a wolf at Lamar Valley, much to our delight! Near Mount Washburn, we spotted our only grizzly bear. Our biggest mistake was not bringing any binoculars on this trip. A very kind couple had a telescope set up on a tripod on the side of the road and allowed our family to take turns to see the bear. This was a highlight for our kids. Other animals we saw were moose and trumpeter swans.
Other areas of beauty include the Norris Geyser Basin, the Grand Canyon of Yellowstone, and my children's favorite, the Artists' Paintpots. Seeing Old Faithful erupt right at 4:26 pm was also a thrill for us. We stopped at Fairy Falls to hike to see the Grand Prismatic Spring, the rainbow colored hot spring that is so ubiquitously photographed at the park. While it was a nice jaunt, the spring is not really visible and there was no overlook to take a proper aerial view picture. We probably should have parked closer to the Midway Geyer Basin to get a closer look.
If you have time, pay the extra money when you enter the park to also visit Grand Teton National Park. Although it's not a WHS, it's beautiful in its own right and the mountains are spectacular. We did spend most of our time at Yellowstone and since we only planned for a week, we were on the go a lot. if you prefer taking it easy, you should budget for more time. It really is a huge park with so much to see and do!