Map of Aegina Greece
On the Aegina Greece map you can see that ferries go between Aegina and Piraeus. If you're staying in Athens you can take the 'three islands' tour, a one day boat trip which visits the islands of Poros, Aegina and Hydra. It's not a bad way to spend a day, although you definitely feel like a package tourist. On Aegina you only get to either stay in the town and run up to the Temple of Apollo, or take the bus and spend a limited amount of time at the Temple of Aphaia.
For me the island has enough to see to warrant a day there. You can get a ferry from Piraeus (it's easy to get to from the centre of Athens by using the Metro) fairly early, and catch a late one back. That gives you most of the day on the island.
There is also a ferry connection between Poros and Aegina, but you'll need to check ferry timetables which can vary by the season. Another connection is from the Peloponnese port of Methana. (Just zoom out on the map and you'll see the ferry links.)
Buses run frequently between the main town and Agia Marina via Paleohora. The journey takes about 30 minutes, and the bus also calls at the Temple of Aphaia. If you're visiting from Athens and plan you day well, you could visit both the site at Colonna and the Temple of Aphaia, have a few hours at Agia Marina, and return for a late afternoon meal before catching the ferry back.
The main town is attractive and has a number of good tavernas and restaurants.
There is also the option of hiring a car or jeep for the day. Scooters and mopeds are also available, but you wouldn't catch me on one...
The map shows you the island and its towns (just hover your mouse over the red markers). You can use the controls on the left to zoom in and out of the map, and you can move the map around by holding down your mouse button and moving the mouse around. The "Satellite" view isn't very inspiring as the resolution isn't that great.