We past this weekend in Hydra.
Unfortunately, I cannot go to vacation this summer. To compensate it somehow, we went to Hydra just for 2 nights/2days.
The Hydra town is lovely, but there is few things to see; two or three small museums and pretty much that is it. Beaches were mediocre by Greek islands' standard - mind that Greek islands' standard is very high -, but considering it takes 90 min from Piraeus port, we thought it is all right.
Down side is that it costs 50 euros to go and return from there and the hotels/rooms are pricy, if you find vecancy, especially during the weekends in summer.
We spent about 400 euros (i.e. 200 euros P/P) for this 2-day vacation. Not so good, not so bad. It was just all right.