Cape Sounion is a beach that is south of Athens, known for its beautiful hotels and luxury restaurants. I've attended so many weddings in the area, as it is one of the most popular destinations for families that live in the Athens or mainland area. One of the best parts of the area is that you can catch a great view of the Temple of Poseidon, here is what it looks like:
Lavrio is the coastal town in the southeastern part of Attica which is the region where Athens is located. This is a great place to visit if you want to go to restaurants and shop around, but also be by the water. Personally, I get pretty tired of staying in downtown Athens because there's not much of a breeze and I can't get to the beach very quickly. This is only super easy if you're renting a car, otherwise you can get to the beach or visit closer beach areas like Glyfada.