Yellowstone National Park, WY
You'll have to elbow your way through large crowds to see Old Faithful, located in the park's Geyser Country. But it's worth the hassle. Imagine the look on your kids' faces when hot water suddenly shoots hundreds of feet into the air.
Although this area is home to many geysers, Old Faithful is the one everybody comes to see. It spits steaming water into the sky every 81 minutes. You could set your watch by it. And don't worry about a no-show. Old Faithful has rarely missed a "performance" in more than 120 years.
Equally spectacular is lesser-known Steamboat, the world's tallest geyser. Located in Norris Geyser Basin -- the hottest, oldest, and most active of Yellowstone's thermal zones -- Steamboat can be fairly unpredictable. But when it blows, it really blows; eruptions can reach 400 feet and last up to 12 hours.
Don't miss a walk along the steamy Midway Geyser Basin boardwalk to Yellowstone's largest hot spring, Grand Prismatic Spring. This 370-foot-diameter, deep blue pool is ringed with yellow-and orange-colored bacteria, giving it a cool prism effect.