It might be too cool to swim in the sea or pools at that time of year.
Mykonos is beautiful and I am sure its suitable for a couple of your age. It has some nightlife but its not lairy and there are great restaurants and shops. I think its becoming popular with cruise companies niw though and certain parts of the island get crowded when the ships are in.
I found Mykonos expensive compared to other parts of Greece. I have never been to Crete but its on my wish list and I would chose Chania. I also love the look of Santorini but I think Mykonos has nicer beaches.
For a typical Greek feel, loads of rooftop restaurants, lovely beaches, relaxing nightlife and nice shops to potter around try Lindos in Rhodes. Try the Exclusively Lindos website (Sorry maybe not helpful if you’re looking at Mykonos!)
Mykonos is probably the most popular island. It is beautiful, architecturally preserved, long sandy beaches and incredible sunsets. It has a lot of shopping and is expensive as it is popular with the rich set and it has a lot of bars as it is popular with the party set.
If you like a busy, fun island then it's the one for you. If you want something quieter then try Sifnos, Mylos, Serifos, etc. If you want something bigger with more archeological sites then Crete is the place.
Mykonos always used to have a reputation as a party island. I don't know whether that has changed. Crete is wonderful. We have been twice - both times in May, and combined lazing around with sightseeing.
Last time we went we stayed at the Castello Boutique Hotel. I can't recommend it highly enough. Everything about it ticked all the right boxes - from the moment we walked into the hotel and were warmly greeted by the staff to the rooms, public areas and food. Having just checked on TripAdvisor they are still getting it right (we went 3 years ago.