Spetses is a very nice little island where you get the choice of just relaxing, going out at night and visiting some interesting sites. It is small enough to get around by bike in 4-5 hours (the road is not the best, though) and there are very few cars. After 2AM there is a ban on driving mopeds, but even until then it is quiet here.
The people here are very friendly, even though the island has been visited by holidaymakers for many years. A lot of Athenians also have summer houses here since it is quite close to the capital.