Poros Island
Don't mix up with Paros, these are two different islands! Poros is not far from Athens, and belongs to Saronic islands.
The Poros island itself is very small. The most popular transport here is motorbike, because there are many places where you can't get by car. But the island is very green - hills are covered with pine trees. There are not much historical places (but they are easy to rich if you rent a car in Galatas and go to Napflio, for example). If you love nature, tranquility and clean water Poros island is definitely for you. And if you search an island close to Athens, that could be a really good choice.
We came to Athens during the strike. It seems, strikes in Greece are normal thing. So before your trip check if there is no strike is planned. We found out about strike just several hours before the flight, and it seemed like a trouble. In that case strike meant that nor public transport nor ferries are working. As far as our travel plans involved islands, it seemed there is no sense to go. But we decided to go anyway and to find some way out.