After the Acropolis I would put the Panathenaic Stadium as my favourite site in Athens. I deliberately planned my visit for early in the morning to beat the heat (there is no shade) and apparently the crowds because it wasn’t at all busy while I was there, but a queue was starting to build up as I was leaving. An adult ticket cost €10 and included a very good audio guide that came in the form of an old style mobile phone. Note that you can only buy admission tickets in person from the entrance booth.
Louise Jayne's Blog 