Crete

That's a perfect amount of time for Crete, especially if you keep to the Western half (which is supposedly the best). We did an 8 day trip a couple years ago and loved it. I will say I don't think I loved it quite as much as the Cyclades islands but pretty close. It certainly has all the things you say you are looking for.

Here is the trip report I wrote about that trip. https://www.fodors.com/community/europe/a-week-in-western-crete-with-photos-1135170/

And here are the photos - https://andiamo.zenfolio.com/p18934947

Nice Photos Isabel you got around.
Cyn you do not have sufficient time to do all of Crete. you need to chose one side of the island and focus your attention there as Isabel did.
We too choose the west although there is lots to do in the East you simply don't have sufficient time to do the whole island.
If you choose the west consider flying to Chania the old town and harbour will allow you to step back in time.
Chania http://www.flickr.com/photos/stanbr54/sets/72157632117917828/
Chania May 2016 https://www.flickr.com/photos/stanbr54/albums/72157671465596275

With 10 days you probably have sufficient time to visit some of the highlights like Balos Gramovossa and Elafonisi and perhaps Ancient Aptera.
Gramovossa, Balos and Elafonissi https://www.flickr.com/photos/stanbr54/albums/72157698932695812

Ancient Aptera, Melidoni Cave and village. https://www.flickr.com/photos/stanbr54/albums/72157672118379637

Later you may want to visit some of the remote beach villages of the south coast which are connected by ferry. You will be planning your return trip a day after visiting your first village.
Ferry trip Libyan Sea https://www.flickr.com/photos/stanbr54/sets/72157653434073393
Crete Libyan Sea villages https://www.flickr.com/photos/stanbr54/sets/72157645487373222/

Thanks stanbr

I was going to suggest that cyn you click on stanb's name to find all sorts of useful info on Crete (and other places in Greece) but then I realized a lot of that is on a different forum :) Fodors forum used to be very 'healthy' with enormous amounts of great info, stanb just being one great source. That forum is now dying but all the old planning posts and trip reports are still there. I've only been over here for a short while but there seem to be lots of good advice here too but perhaps checking there will bring up some useful information. In any event, take stanbr's advice, he was a major help to me planning my trip, and one of the reasons it came out as good as it did.

You definitely should stay in Chania. It's location gives you access to the western beaches of Elafonisi and Falsarna as well as day trips to historic sites in W. Crete. One day you should go to see Knossos in Heraklion, otherwise stay on the western side.

I went to Greece in October 2018. I went to Crete. I was only there for 2 nights. Anybody who says that you could have a nice time spending 7 to ten nights in Crete is right. In Heraklion I saw the archaeology museum, Crete history museum, and I went inside the Koules Venetian fortress; I saw Knossos, and Chania because others recommended it. Don’t pay attention to anybody who says that Heraklion is a city you should spend little time in. Both towns are different. Neither one is objectively better or worse than the other. Chania is over-rated on this forum and and Herklion is under-rated. The only good outdoor scenery in Chania is the crowded touristy area by the harbor and Venetian fortress and mosque of the janissaries. Walking through Heraklion can potentially occupy you for longer than Chania. But I did like the maritime museum in Chania. I bought the Rough Guide to Crete. Rick Steves does not cover Crete. Crete seems to lack Greek architecture from around the time of the Parthenon. The Island does have Venetian fortresses and Minoan ruins in multiple locations.