Let's start with some of the oldest items in the museum. The Thira frescos, located on the upper floor, were uncovered on the Cycladic island of Thira, also known as Santorini, and are world-famous. They date to approximately 3500 BCE, and have survived so well principally because they were buried by a devastating volcanic explosion that covered the ancient settlement in ash.
This one, known as "Antelopes" is one of about four currently on display. Apparently, the residents of Santorini want the frescoes returned to the island so it's not clear how much longer they will remain on display in Athens.