Yellowstone National Park is more than just Old Faithful. As America's first national park, Yellowstone has classic park architecture, herds of wildlife, and hundreds of miles of hiking trails. For most visitors, Yellowstone National Park is all about the different areas of the park - Old Faithful, Mammoth, Canyon, Norris, Lake, Tower, Roosevelt - accessible by the Grand Loop road and the feeder roads to the various Yellowstone gateway towns.
Mammoth and Old Faithful have the most accessible geysers and thermal areas. All of Yellowstone is geologically active, as most of the park is actually in a volcanic caldera. The caldera is not easily visible, because an eruption several hundred thousand years ago destroyed any signs. The new Canyon visitor center's focus is park geology, and park rangers there can answer questions about how the park was formed. A large 3-d park map shows the extent of the caldera.
Some of the best encounters in Yellowstone are the least expected. You may just be driving down a park road to find an enormous traffic jam - this usually means bears, moose, or wolves on the side of the road. Other times, you may find yourself witnessing a rare eruption of one of the park's less active thermal features.