Well, with a baby in a backpack, I guess your hiking range depends on how strong the backpack carriers are! Our favorite hike was to Fairy Falls and the Imperial Geyser area -- about three miles each way, not hilly or difficult, and you get to have your own private geyser show, which teenaged boys would really enjoy -- it is noisy and violent. Just be careful where you step or sit, because some of the ground is hot, and the hot stuff is dangerously thin ground:
In that shot, I tried to time it so that the geyser would be going off during the shot, using the self timer on the camera mounted on a tripod. I could not catch the geyser at full height -- but this is what it looked like at its peak:
The falls are pretty cool, too -- you can get some idea of the scale by comparison to the size of my wife, in the lower right corner of the shot:
You could do this hike with a baby, if the baby will nap. Also, there are sandy places near the geyser that are safe for a crawling baby (test it with your hand first) -- the sandy places are buffalo wallows, where the bison come to scratch their backs in the sand.
If you are interested in more detail, here is a link to our blog post -- there is a lot of stuff in this post about other areas (like the Tetons), but just scroll down to the Yellowstone area:
Yellowstone blog post
