Lavrio is one of the best beachside places to visit near Athens. It's a bit of a drive so you'll need to rent a car or take a bus. There are excellent beaches and tavernas there and you can also visit the neighbouring town of Sounio where the famous Grecotel hotel is located. Since it is a tourist location, Lavrio has a bunch of water sport activities such as snorkelling, scuba diving, boat rentals and many more. it's a great two day trip if you're wanting to get away from the city life.