Cabrillo National Monument, San Diego, California (click here for more info)

Juan Rodriguez Cabrillo was the first European to set foot on the west coast of the United States. This National Monument is the most southwesterly spot in the contiguous United States.

While I was there, I got a text message from Verizon saying "Welcome to Mexico" with instructions on how to place a call within Mexico. Later while driving, I saw signs that said "Welcome to California". I'll admit I have no sense of direction but I know I never left the states!