Most people visiting Potsdam are there for one reason—to view Frederick the Great's palaces. But in addition to the grandiose architecture, you can actually visit Frederick the Great's grave.
Frederick the Great wanted to be buried on the grounds of Schloss Sanssouci, his summer palace. As a relatively benevolent Prussian emperor, he was much revered by people around the world. One of his accomplishments, though, is a bit odd for such a cultured emperor. He's largely responsible for introducing the potato crop to the region. During his reign, he made a special point to promote the potato: Rumor has it that he actually stationed guards around the crops to make them appear valuable and therefore more desirable.
As a sign of respect today, visitors place potatoes on his tombstone. Leave it to Germany to show respect with such an unusual symbol!