The claim that the Wall is 300 miles long is further supported in Game of Thrones: A Telltale Games Series, where Britt Warrick says to Gared Tuttle a 300-mile wall, and you're still in my way. The game is canon to the show so it is possible that Sam was wrong.
One of the most respected collections of museums in the world, the Smithsonian Institute is an inspiring attraction for kids, teens and adults. Comprised of 19 different museums, art galleries and research centers, the Smithsonian collection is centered around the National Mall - and admission is always free. The most popular Smithsonian museums include the National Archives, National Zoo, National Museum of Natural History and the National Air and Space Museum.
While it's not part of the Smithsonian, the National Gallery of Art is a worthy diversion set amid the museums. In addition to a world-class collection, the gallery has the best gelato I've had this side of the Atlantic. In the lower level, there's an espresso and gelato bar that's well worth a respite.