Greek Islands (Cyclades) from Seattle. What do you think of this itinerary? Mid-May

Crete is huge and it takes a bare minimum of a week with a rental car just to scratch the surface. If Crete is a "must" then eliminate either Santorini or Naxos. You should know that from London there are direct EasyJet flights to Santorini as well as to Crete, so you could easily fly into one and out from the other, eliminating time-wasting backtracking in the process.

It appears you have 10 days in Greece and some of the days will be spent on a ferry which will eat up a lot of time. I would suggest picking one or two at the most islands and spend your time there. As stated Crete is a large island and deserves a lot more time to experience what it has to offer. Naxos is one of my favorite islands and offers some of the best of Old & New World Greece. Santorini while visually stunning is one of the most touristed, crowded and expensive islands in Greece. If it was me I spend all my time on Crete or at the most split the time between Crete & Naxos remembering the time involved with being on a ferry. Flying to various islands is one of the quickest ways to arrive and will save a lot of time. However not all islands have connections between them.

I agree with the others and would cut your itinerary and just visit Crete. Santorini is the single most over-rated Greek island IMO.

You are travelling very early in the season, so I would avoid ferries possibly being cancelled due to adverse weather and stay put on Crete. Greek ferry workers are notorious for striking and I would not risk taking three ferries in a week! There is more than enough to occupy you on Crete.

Ditto to Jennifer's comment about the ferries.