One of the hidden gems is the Benaki Museum of Greek Culture in Athens. Located just off Syntagama Square, the museum houses on the the largest collections of Greek art. From their folk costumes to their Minoan pottery, there is something for everyone to see.
Getting There

The best way to reach the museum is by foot. Using Syntagma Square as your landmark, if you walk towards the Greek Parliament building, continue on until you are across the street from the National Gardens. You may even see signs for the Lyceum of Aristotle.
Tickets

The museum is open everyday except Tuesdays and national holidays. The hours of operation will be shorter on the weekends. Tickets range in price. I would recommend beginning with the permanent collections and seeing how you feel after finishing your tour of the museum. Allowing one to two hours should give you enough time to see everything.
Full admission begins at 12 Euros, but there are discounted tickets available for students, seniors, and archaeologists. The list of prices as of June 2021 is being updated in anticipation of a summer reopening.
Tickets may be purchased online or in person.
Collections

The collections of the Benaki Museum begin on the ground floor with the Prehistoric, ancient, and Roman art. There are 36 rooms in total. From the ground floor you chronologically move through the history of Greece. You will find items that are also Byzantine, Islamic and Asian. Towards the more contemporary area of the museum, there were quite a few religious items. They provide a unique incite into the modern Greek life. I enjoyed seeing the collection of folk costumes and contemporary art. Of particular interest are the reconstruction of reception rooms found in stately mansions in Greek Macedonia in the mid 18th century. The display cases on the ground floor are better labeled than those on the upper floors, but you can always ask many of the guides for further information.
A free virtual tour is offered here.
Shopping and DIning

The Benaki Museum features a small shop on the ground floor with items that range drastically in price from postcards to designer jewelry. It is worth stopping in to see what they carry.

On the second floor of the museum, there is a small cafe that features a nice view of the city and offers a respite from all the walking you will do during your visit. The food can be a bit pricey, but is well presented and worth the splurge. Try to sit outside if possible.
Wrapping Up
As the museum reopens to the public, this page will be updated with further information such as ticket prices and hours of operation. In the meantime, enjoy a virtual tour of the museum and consider taking a visit. It is well worth it.