Museum of the Modern Olympic Games
This small museum off of the main street in Olympia city, hosts a bunch of artifacts from the modern Olympics, including the silk-lined box in which they transported Pierre de Coubertin's heart to be buried in Olympia in 1939.
Lots of photos and documents from the revival period and a nice summary of each host city for all summer and some of winter games.
Tripadvisor members state about this museum: "If you go to Olympia you must go here. It's part of the deal"!