Aegina is accessible from Athens by ferry or hydrofoil from Piraeus port. Conventional ferries take about 70 minutes and run frequently throughout the day; the faster "Flying Dolphin" hydrofoils cut the journey to around 35–40 minutes. Both options are cheap and reliable, and you can buy tickets at the harbour. From central Athens, Piraeus Metro station (Line 1, the green line) takes you directly to the ferry terminal.
Day trips from Athens are entirely practical — the first ferries leave early and the last ones return late in the evening. Alternatively, stay overnight or for a few days; Aegina has a reasonable range of accommodation in the main town and at the beach resorts. There are no flights to the island.