Getting into the city center (Syntagma square) on Saturday morning by taxi should be quick. Getting back to the airport in early afternoon could be a little longer. I would personnally allocate two hours, two and a half hours at the utmost for the round trip. Even if you are not checked through to Sydney, this does leave you some time for sightseeing.

Staying at the airport does not offer anything meaningful to do : spending time in a lounge or at the Sofitel's ground floor brasserie (the upscale restaurant is only open for dinner) is not very exciting.