Of course you'll do Yellowstone's Old Faithful. But you also should head 7 miles north and visit Grand Prismatic Spring. It's the largest spring in the park, about 300 feet across, and its boiling deep blue waters, fringed with yellow and orange crust, give off billows of steam that envelop you as you pass on a raised boardwalk. If you have time, climb one of the two neighboring ridges for a bird's-eye view.
In honor of this year's National Park Service centennial, the Travel section is posting 100 park travel ideas and tips based on trips staff travel writer Christopher Reynolds has taken, along with photo-op advice from Times photographer Mark Boster. We'll post one per day through Dec. 31.
Follow Reynolds on Twitter: @MrCSReynolds
See travel videos by Reynolds from around the world.