With the 2010 Winter Olympics in progress, naturally the site of the revival of the modern Olympics comes to mind.  The 1896 Summer Olympics were held here; also, check out the Athens Olympic Stadium and sports complex constructed for the 2004 Summer Olympics.  If your tastes run more towards archaeology, there's the Archaeological Museum , where you can see the Antikythera Mechanism among many other artifacts.  Count on a geek to be interested in ancient mechanical computers...