For tourists the most popular islands include Aigina, Poros and Ydra. Aigina is the site of the Temple of Aphaia, considered the most well-preserved temple in all of Greece. (see photo below)
Both Poras and Ydra are famed for their quaint villages and casual atmosphere, and on Ydra, no motorized vehicles are allowed so bikes and donkeys provide the local transportation.
All of the islands are accessible by a reliable ferry service from the mainland, and one-day, three-island cruises are recommended for those with limited time; they're affordable, they're fun and they're very Greek!