You are going to learn that a "cruise of the islands" is basically a sea voyage for 80% of the time, NOT a true visit to Greek Isles. our time onshore will be a rushed & crowded scene, surrounded by your fellow-cruisers most of the time --- sometimes 3-4 cruise ships dock at same port at same time, so up to 8,000 cruisers can be flocking thru the narrow streets, cramming on to the same busses. You are gong to have to plan carefully (and in some cases, spend $$$) to get away from the mob scene. I realize you don't want to hear this, but better to know beforehand, so you can strategize to improve your experience.
First of all, Start Googling: you might try a very specific question "6 hours on (name of island) on a budget: Help!" then you will get info YOU can use, not suggestions for posh deluxe private tours, but answers that budgeteer have found. It will help if you do some homework on your own, i.e., find maps of each island, look at guidebooks, not expecting to learn eerything from a travel forum.
I've never been to Corfu, but can give you my insights into the other 2 islands. Expect the time ashore to be MUCH less than listed becaue you have to factor in the time to disembark and re-board your ship (sometimes loooong lines).
SANTORINI -- The Cruise Liners really rip you off on this one, for an improtant reason. All Cruise ships do not dock at the modern port, where you can step onto the pier and get a taxi or bus on a road up to the Top of the cliff. Instead they moor out the caldera , and passengers must be brought in on launches to the OLD plier... and then must get up to the top via cable car, which is quite limited. You can imagine if 2,000 people want to come ashore, this process can take well over an hour. AND the Cruise line puts excursion-buyers At the FRONT of the LINE. So unfair! So resign yourself... and ask (bribe?) a crew member on how to get onto the first launch AFTER the excursion group. Don't count on getting to top of the cliff, into Fira Town, until at least 3 pm, maybe later. And because of cable-car limitations, you'd better be back in line there by 9 pm. So basically you have 6 hours. What to do?
The DONTS ==
• DON'T trek to black beaches; they are volcanic GRIT, burning hot in August, like a diner grill! Wait til Mykonos for Swims
• DONT Count on Museum, alas. It's lovely & tells all about ancient volcano event, but it closes at 3 pm
• DONT attempt sunset at OIA!! It's Madness; worse than Times Square at Rush Hour -- endless ballyhoo about "OIA Sunset" has created a monster! The rim parapets are JAMMED with thousands of people nightly, Sunset won't be until 8 PM, then they all try to get onto the same bus, impossible ... you would miss your ship! Don't believe? Here are pictures! http://tinyurl.com/lnquk6h Don't worry; I have better answers.
The DO's -- USE this MAP -- https://dessylovetravelling.files.wordpress.com/2012/09/map-of-fira-santorini-greece.jpg
• When u get off cable car (N on map) locate STANI RESTAURANT -- it's on that same lane -- ask at any store. Here is link. http://www.stani.gr Go to top-floor terrace and see STUNNING view. Pick a table and make a reservation for 7 PM -- they WILL honor it (I have done this before). They have authentic Greek food at decent prices & like kids (I see on another thread you are taking "kids" along -- for hevvins sake, when you want good advice tell us important stuff like this )
• Go get snacks, And walk to BUS Yard ("O" on Map) to get public bus to OIA ... don't arrive before 4 pm, that is when cruise mobs start thinning out... From 4-6, least crowded time in OIA... you can wander around & take photos Keep track of Bus schedule!! Catch bus by 6: 15, to get back to Fira Town in time to stroll along promenade past Cathedral, and get to STANI for nice meal.
• After Sunset, you'll have time to buy souvenirs, and be in the cable-car line by 9 pm.
For Mykonos see messge #2