Things to do in Ermioni are not many as the region keeps a lower, family profile in tourism. The only organized activity in Ermioni is diving. In fact, there are a couple of diving centers that offer courses and organize diving trips to interesting reefs and uninhabited islets of the Saronic Gulf.
For more active holidays, head to the close seaside town of Porto Heli, where there are a couple of water sports centers with fun games. The lake of Ververoda in Porto Heli is also popular for water skiing.