For those saying Plymouth Rock and the Alamo are underwhelming are missing the point of the two places. Plymouth Rock has been chipped away, moved, and erroded for 300 years. Only a portion of what is believed to be the the original stone remains. What is more important is what happened (or supposedly happened) over the subsequent year. Plymouth Rock comemorates the ordeal of the original group of Pilgrims and the first Thanksgiving. Without it, we would not have two days off from work, Family Gossip, Pants with expandable waists, Thursday Morning High School football, Thursday Afternoon NFL Football, or Black Friday.
Similar with the Alamo. The original structure was much bigger, and was never meant to withstand a fire fight beyond what the area Native Americans could muster. Beyond that, it comemorates the first battle of the Texan Revolution which ultimately gave Texas its independence.
I thought Old Faithful was cool only because they can pretty much the occurance of an untimable natural phenomenon. As far as other natural wonders, the Grand Canyon is indeed awesome, as is Mt. St. Helens (the crator is in the side of the mountain, not the top).
Man-Made: I enjoyed my time at the Gateway Arch, the 6th Floor Museum in Dallas, and anything in Washington D.C. whenever I go.
As I've said in other post, I was barely whelmed by Mt. Rushmore. On the other hand it is not complete as originally intended.