The island of Poros is about one hours drive away from Ermioni following the coastline and passing opposite Hydra island. Galatas village is the place where you get the ferry from or just a boat without your car. It is only a 5 minutes trip from Galatas to Poros.
Nice, very green island and busy at times. You will find tavernas and all types of beaches but in a more touristy way than Ermioni. If you like you can continue to Methana peninsular which is another 30 minutes drive after Galatas village.