UNIQUE Greek Island Experience

A great resource for you as you plan your trip to Greece is Matt Barrett’s Greece. He has a section on the Greek Islands. www.greecetravel.com
This is an extremely helpful website for planning a trip such as yours.

Among the twenty or so Cyclades islands there are a few which are relatively less invaded in summer.

Kythnos (1 ferry 3 times a week, 3 hours crossing time)

https://www.visitgreece.gr/islands/cyclades/kythnos/

Serifos (3 to 4 ferries per day, 2 to 4 hours crossing time)

https://www.visitgreece.gr/islands/cyclades/serifos/

Kimolos (3 direct ferries per week, 4 to 5 hours crossing time or reachable every day via Milos with 5 ferries per day Piraeus→Milos)

https://www.visitgreece.gr/islands/cyclades/kimolos/

Folegandros (1 to 2 ferries per day, 4 hours crossing time)

https://www.visitgreece.gr/islands/cyclades/folegandros/

Amorgos (1 ferry per day, 7 hours crossing time)

https://www.visitgreece.gr/islands/cyclades/amorgos/

Tinos (7 to 8 ferries per day, 2 to 4 hours crossing time from Rafina port)

https://www.visitgreece.gr/islands/cyclades/tinos/

Sifnos (3 to 4 ferries per day, 3 to 5 hours crossing time)

https://www.visitgreece.gr/islands/cyclades/sifnos/

Looking for less busy and popular and easy to reach is not without compensation. None of these islands have an airport, so you will have to take a ferry.

Outside the Cyclades there are also for example Kythira and Limnos which have an airport with direct flights from Athens

Kythira (Ionian Islands)

https://www.visitgreece.gr/islands/ionian-islands/kythera/

Limnos (or Lemnos) North Aegean Sea

https://www.visitgreece.gr/islands/north-aegean-islands/limnos/

Wow JoLui! Great info, I’m bookmarking this post!

July will be the height of tourist season so finding an island that isn't filled with tourist will be challenging.

Check out my photos of these three which have airports making it quick to get to:

Ikaria
https://tinyurl.com/y4e5dsj6

Kythira:
https://tinyurl.com/Kythira-Greece

Astypalea:
https://tinyurl.com/y7mdg7rv

This next one is a gem and one of my top three favorites but you'll need a ferry to get to:
Tinos
https://tinyurl.com/y6w5ncb8

Lots of other islands that may work for you but some of them are more challenging to get to and with your short time frame you may want something that is quick and easy to get to.