Tony
I think Olympia is too far for a one day trip.
Sounio is possible as a half day trip. The setting and temple are really nice, but the journey is nothing special.
Mycenae is possible as part of the day tour of the Argolid, together with Nafplio and Epidavrus. It would also be easily possible by scheduled bus.
Another possibility which,as far as I know, has no organised tours to it, is ancient Corinth. There is a lot to see at the Roman site and acrocorinth, the older fortress above, is huge.
We were there on a sunny December day a few years ago and had acrocorinth entirely to ourselves. Walk up high enough and you can see both ends of the Corinth Canal. You also pass over this on the way there.
There are also a couple of things on the outskirts of Athens that are only half days, but might be worth considering.
The monastery at Kessariani is a nice building and set into the hillside above Athens. The 224 bus runs past the Hilton and you can catch this to the end of the line. The monastery is about 30 minutes walk further up, initially walking past the cemetery. Once you cross under the motorway you can cut up onto the shoulder of Mount Immitos, and you get great views back over the Acropolis.
Ancient Elefsis is also a nice site, and there are some nice little places to eat on the coast there. Local buses A16, B16 and E16 all go there from Eleftherias Square on Piraeus Street.
BTW, it might be worth your getting a week travel pass. 10 euros and good for buses, trams and metros. The trips to Kessariani and Elefsis would both be covered by this, but note that trips to and from the airport aren't.
Cheers
Alan
PS I've never heard of the spa of Epidsos. Do you know whereabouts it is?
