There are regular ferry and hydrofoil services from Piraeus port to the Saronic Gulf islands especially during the tourist season from April to October each year. If your time is limited you can book the one-day cruise to Aegina, Poros and Hydra. This is a daily cruise which starts around 8.00 am and returns to Athens at 7.00 pm and includes lunch on board and stops at all 3 islands. There are also ferries and hydrofoils which link these islands with each other.
One should note that on the islands of Idra and Spetses no cars are allowed so the best way of getting around these islands is on foot or by boat. As for the other islands you can use the local buses or taxis to discover the little villages or numerous beaches.