Yellowstone is full of wildlife, but most of those animals live hidden lives most of the time -- especially smaller animals that fear being preyed upon by the larger carnivores. In our hikes around the park in September, we saw evidence of ground squirrels, badgers, and beavers, but we never spotted any of those animals themselves.
But if you'd stayed there silent and downwind long enough, you almost certainly would have seen them.
Humans on hikes are pretty noisy, go at a steady pace, don't stop all that much compared to other animals, or for long, don't change directions much. If you want to see a lot of wildlife on a hike you must wander about slowly and quietly, sit somewhat hidden and stay still gazing about for five or more minutes at a time, change directions and gaits frequently, be away from too-frequent intrusion by those who aren't acting that way. You can't dawdle about an afternoon in Yellowstone behaving in this fashion; you'll accidentally sneak up on a bear and have a bad time.
I think wolves on the move and purposeful wouldn't see much, but lazy wolves goofing off would.
I like that so many suggested ground squirrels. I think yellow bellied marmots would have nearly the same model and AI, just different size. You'd have to hunt them by finding their holes and waiting for them to come out, which would be different (but maybe not what people who aren't me think is fun?) and one could give a pretty good natural history lesson in showing when they are active and in what numbers, because they hibernate in winter and also stay underground all the time when it's very dry and hot, so there are lots of times when you will not see one for months or weeks, and weeks when all the juveniles are emerging and there are suddenly a lot of them. No idea where other people put that on the fun-meter, much less the game-creation work-o-meter.