Of all the islands within reach of Athens, Aegina is the most practical for a day trip. The high-speed catamarans from Piraeus take around 40 minutes, conventional ferries around an hour and ten. There are departures throughout the day, so you can sleep in, spend six or seven hours on the island, and be back in Athens for dinner without rushing.
The standard fast-ferry operators depart from Gate E8 or E9 in Piraeus, a short metro ride from central Athens on Line 1 (green line) to the Piraeus terminus. Buy tickets online the evening before, especially on summer weekends when boats fill up. A return ticket on the fast ferry costs around 30 euros per person at current rates.
If you have only one day, the best use of it is to rent a scooter or bicycle at the port immediately after arrival, ride to the Temple of Aphaia (about 12 kilometres, signposted), and then loop back via the village of Mesagros for a pistachio stop and lunch at a taverna in Perdika. The loop takes three to four hours at a relaxed pace and covers the island's highlights without backtracking.
Alternatively, if you would rather not ride a scooter, buses connect the port to Agia Marina and the Temple of Aphaia. Taxis are also plentiful and reasonably priced by Greek island standards.
The last fast ferry back to Piraeus usually departs around 8 or 9 in the evening — check the schedule when you buy your outbound ticket so you are not caught out.
Photo: Ank Kumar / Wikimedia Commons (CC BY-SA 4.0)