Crazy Horse Monument is very cool and HUGE! It is worth a google if you don't know much about it. Sadly we didn't have much time here but I'm glad we went.
Fun Fact: The heads of the 4 presidents would fit into the head of the Crazy Horse carving..interesting and puts the size into perspective.
Not so much a monument BUT the town of Deadwood is fun b/c of the HBO show but also b/c of the Historic West.