The Stavros Niarchos cultural center is a great place to bring kids and the family. The architecture is amazing and there is a beautiful area to walk outside and enjoy the nature and views. The center is known for sustainability and innovation, and even has its own sustainability hub. There are multiple exhibitions going on throughout the year that showcase Greek Culture and modern and historical art. Here is the link to the foundation https://www.snfcc.org/en
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.