55km (31 nautical miles) SW of Piraeus
Poros shares the gentle, rolling landscape of the adjacent Peloponnesian coastline, and has several good beaches, some decent tavernas, and lively summer nightlife. If that sounds like lukewarm praise, it may be. For me, Poros lacks the sense of being a world unto itself that makes most Greek islands so delicious to visit. And, in July and August, the island virtually sinks under the weight of day-trippers and tour groups.
As someone once said, "geography is destiny." Poros (the word means "straits" or "ford") is separated from the Peloponnese by a narrow channel only 370m (1,214 ft.). It's so easy to reach from the mainland that weekending Athenians and many tourists flock here each summer. A car ferry across the straits from Galatas to Poros town leaves about every 20 minutes in summer, which means there are a lot of cars here.
Technically, Poros is separated by a narrow canal into two islands: little Sferia, where Poros town is, and larger Kalavria, where everything else is. If you wish, you can use Poros as a base for visiting the nearby attractions on the mainland, including Epidaurus, ancient Troezen (modern Trizina), and the lemon groves of Limonodassos. In a long day trip, you can visit Nafplion (Nafplio) and Epidauros; in a very long day trip, you could also see Mycenae and Tiryns.