If you are planning to stop by Athens, here's a quick itinerary for your trip:

Hop on/Hop off BLUE line (athens, piraeus coastline)

street food tasting tour (A HUGE MUST- book through getyourguide)

Acropolis and Parthenon - don’t do a tour, it’s a lot nicer to do on your own

Acropolis Museum (ABSOLUTE MUST!! but it’s freezing cold there so bring a sweater)

Day trip to Delphi and Arachova (long drive, but 100% worth it)

Ancient Agora, Roman Agora

Temple of Poseidon (pretty but sorta overpriced and overrated)

Walks all around Plaka, specifically Ermou Street

Monastiraki square

Syntagma square

lots of the free walking tours, they’re a lot of fun!!!

restaurant MUST: Tudor Hall (go in evening, has view of Acropolis at night) and CTC Gastronomy (weird neighborhood street, but the food is so much fun and an experience)

If you plan on doing some island hopping, here's a few suggestions:

Milos - very calm, beautiful beaches, lots of beaches (must-see Sarakiniko Beach). Go for a drive and find the original statue of Aphrodite. Head out to Klima and have dinner at Astakas Cafe Restaurant- best view of the sunset on the island. Be sure to go to the center and walk to the Venetian Castle of Milos.

Paros - Nightlife + relaxation. Stay in Naousa and try to rent an ATV so you can get around the island easily. Santa Maria Beach, Kolybithres Beach, Piso Livadi Beach and then drive into Lefkes town. Make sure to go to Taverna Glafkos for dinner (no reservations, but worth the wait).

Naxos - Rent a fishing boat for the day - comes with food, drinks, music, swimming and fishing! The nicest, most clear water I have ever swam in!!!

Mykonos - full party island! Check out Nammos Village in the evening time. Spend time checking out the town center (easiest way to get around is a taxi, uber is less common). Rent a daybed at Pinky Beach (very reasonable prices). If you want to check out Scorpios Beach Club, you have to make a reservation WAY ahead of time and they will only let you in if you are dressed the right way. Wind mills are cool, but don't necessarily require a full guided tour. Reserve dinner at Mando Restaurant for the best octopus of your life.

Santorini- Sunsets galore! Make sure to watch the sunset from Oia at least one day. Celebrate Masha and George getting married !