I've spent several seasons in the Saronic Gulf, right up to mid October. The weather
is still warm, but evenings can be cool. The area will be much quieter than earlier in the year and tavernas will still be open.
Good places to visit include Palaia Epidavros, anchor
in the bay or tie to the quay, visit the two amphitheatres in the area, the small one that you can walk to through the orange groves and the larger, more famous one that you will need a taxi or bus to visit (well worth it). Aegina is a nice town to wander around but the harbour is notorious for crossed anchors and shouting, you can anchor outside in settled weather
. Opposite Aegina is Angistri which has a couple of nice anchorages
and the Metopi Shoal, good protection from N/NW and clear water
. Poros has numerous anchorages, the best for swimming are to the west, nearer the town it is very murky but excellent holding for visiting the lovely town. Good places to eat are Poseidon, Appagio, To Kypos. Hydra is lovely but very touristy, we went on the 12th May (right at the beginning of the season) and struggled to find a spot to moor. There is an anchorage outside the town if the weather is good, or go to Ermioni and get a hydrofoil over for the day. Ermioni has two areas to moor, north and south, both good, the anchorage to the north is also excellent. There's a market on Thursdays.
If you have more time it is worth going round to the Argolic Gulf, there are some lovely anchorages including Korakonisia and Vivari. Nafplion is a lovely city to visit if you have time.