Helen, I'm sorry, but "packing & unpacking" is the name of the game in island-hopping. Both the idea of "basing" and "daytrips" from Crete or Santorini are not as easy as you'd think. Many people do this in Italy Spain or France but it doesn't apply in Greece, except in specific places. Here is a good map -http://www.explorecrete.com/crete-maps/images/greece.gif (click to enlarge) which can illustrate what I mean.
DAY TRIPS -- There are no day trips to interesting islands from Santorini --- it is too distant from most islands and schedules of regular ferries are not geared to daytrips. Given the schedules, You'd just get there and have to turn around and go back. Ditto for Crete. Ditto for Mykonos, unless you want to see Tinos for a couple of hours. The islands look close together, but looks are deceiving ... and it's not like Spain or Italy where you can jump on a train & get somewhere 50 miles away in an hour. There are no trains in the sea.
To illustrate, find the 2 islands Naxos & Paros on the map. See how close they seem? From a Naxos beach, Paros is just across the way. But to visit via "regular" ferry, youi'd have to walk or bus, or taxi to a pier, wait for a ferry (and the first one isn't until 11 AM, spend an hour on the ferry, and presto! a whole morning is gone. The "regular" ferries returning are either right in the middle of cocktail/dinner time, or late evening. Too much time waiting & travelling, not enough time enjoying.
You have a better chance if you stay on Naxos or Paros --- because (almost alone among all the Cyclades) these 2 islands have a company with Dedicated Excursion Vessels serving both isles. Boats depart around 8 - 8:45 am, and get you back to your "home" island by 6:30 or 7:30 pm. https://visit-cyclades.com/day-cruises-naxos-cruises/ ... this company offers a number of day-trips: a boat will start from Naxos, stop at Paros, then visit Delos/Mykonos from about 10:30-4:30, then return. Also daytrips from Naxos - Paros, Naxos - Antiparos and more. In fact, if you're just going to Mykonos because it's "famous" and you have no special interest in it except friends were there once, you might consider staying on Naxos or Paros and doing a day-trip to Mykonos.
Crete is another matter. The only day trip is to Santorini, and the reviews are bad; a looong trip, a short stay. Crete is a BIG island -- it's 175+ miles long, its reallly a country... to do more than scratch the surface, you need to spend 5 days, and rent a car. And while in crete, you really miss out if you insist on the same hotel, the same bed for 5 nights. It would be like visiting California for a week, driving all over, and returning to your San Francisco hotel every night. 50% of your time would be spent backtracking.
The No-repacking idea, returning to a "Base" nightly. It will work in Naxos or paros for a visit of 4-5 days or so. Not in Santorini, or in Mykonos or in Crete. You stay a few days, and then move on... that's why it's called island-hopping, not island-basing. Believe me, it's not that burdensome. Just keep most of your stuff in the bag, on a baggage rack, hang up a couple of tops & slacks & relax. You don't have to worry about dressing up; nobody does, in the islands; no Heels, jackets or jewelry to fuss with. Try it, you'll like it!