You could spend a month in Crete and not see it all. It is a treasure trove of Minoan ruins, Venetian era cities, mountains, hikes and the sea.
We have been to Crete 8 times each visit a minimum of 2 weeks and a full month the last couple of trips. We have still not seen the far east end of the island.
At this point you will use 4 nights Athens plus 4 nights with friends. That leaves you the 17th until the 27th, 10 nights to have your Crete experience.
Just an aside it is always a good idea to be back in Athens a day before your return flight. Stuff can happen on holidays bad weather flight and ferry delays. Have you considered going to Crete when you arrive in Greece and spend the last 4 nights in Athens at the end of the trip. That way you do not need to get to Athens the night before your return flight because you will be there exploring the city. That gives you an extra night in Crete.
We love west Crete. We make Chania our base. Specifically the old town area and Venetian harbour. I think it is the most beautiful city in Greece. Chania has an airport so you can easily connect to a flight to Chania on the day you arrive.
You can use Chania as a base to explore the west. Ancient site at Aptera has mostly Roman era ruins. Day trips to Gramvossa and Balos lagoon are popular. 14th century fortress and a ship wreck at Gramvossa and one of the nicest beaches anywhere at Balos Lagoon.
Day trips to the pink sand beach at Elafonissi are also popular.
The famous Sameria gorge hike is also a great day trip from Chania.
Other day trips could include Falasarna a huge beach, and perhaps the second Venetian city Rethymnon about an hour from Chania.
A real treat would be to drive (or take the bus) to the small village of Sougia on the south coast of the Libyan sea. Its a get away from it all, location. You can take a ferry along the cost to Agia Roumelli where the Sameria Gorge ends. That gives you an opportunity to do the lower flatter part of the Gorge before taking the ferry back to Sougia or take the second ferry east To Chora Sfakion.
If antiquities is something that interests you head to Heraklion. It is a very busy city but still has a nice central area were the locals spend every evening on their promenade. You can visit the Archaeological museum there then visit the Minoen site at Knossos where much of the stuff in the museum came from. Continue heading south to Phastos another Minoen palace but not restored like Knossos. Keep going further south and you come to my favourite site Gortyz or Gorten. It was the roman capital of Crete.
There is a gated site there however from the site walk from the parking lot along the road through olive groves. There is a whole Roman city buried there. It has been partially excavated but you feel as if you have just discovered it yourselves.
I just realize I probably have become long winded. All that to say it is really easy to fill a couple of weeks on Crete. There is no need to find somewhere else to go after you visit family. Crete has it all.
Here are some images.
Archaeological sites Phaistos, Gortyz and Knossos.
https://www.flickr.com/photos/stanbr54/albums/72157645085297380
Chania 2022. https://www.flickr.com/photos/stanbr54/albums/72177720299646015
And images of a trip to Chania, Sougia then finishing at Plakias a lovely village on the Libyan sea.
Chania Sougia and Plakias 2019
https://www.flickr.com/photos/stanbr54/albums/72157711266062752