|Gables at the Old Faithful Inn, completed in 1904, Yellowstone National Park|
Go to the Old Faithful Inn. You could go to the hotel in Mammoth too, but let's face it, the Old Faithful Inn is the most tradition-soaked, parkitecture-perfect building in Yellowstone.
Bring a bag with a towel, soap, shampoo, etc., unless you want to pay extra.
Walk up to the registration desk and tell the clerk that you wish to take a shower. The clerk will collect $3.45 and direct you up the creaking stairs to a restroom with shower stalls in the back.
The stalls are clean, tiled, and have windows that open to views like the one above. The shower is an old-style pierced disk high overhead. It feels wonderful.
Once clean and dry, stop at the Mezzanine Bar for a Snake River Lager, then carry it outside to one of the benches facing the Old Faithful Geyser and watch for it to erupt.