Greece 3 week itinerary

Travel forums are filled with people who share real experiences and insights tailored to your unique needs, be it the season you’re traveling, the type of transportation you’re using, your budget, and your personal interests, unlike the overly simplistic itineraries generated by AI, which are often as useful as an oyster in planning a trip

Just looking at the first 3 days of this AI-generated itinerary makes it clear to me that it will only lead to disaster.

That’s why I’m here asking. Looking for those real experiences to be shared. Hoping someone helpful comes along.

To begin to answer your questions, you would first need to tell what is the question you asked to the AI.

Did you simply request a 3-week itinerary starting from Thessaloniki?

Did you specify any dates? How many travelers? Any kids ? What are your preferences regarding transportation (rental car, train, bus)? What places /islands do you want to visit? Did you do any research beforehand to choose all these places?

There are so many unrealistic aspects in this itinerary as proposed by the AI ​​that it's impossible to explain everything and to correct it without knowing more about your needs.

Your Cyclades flow is solid — and honestly AI did OK here, because there’s a lot of good info out there for Mykonos/Naxos
Days 12–15 (Mykonos + Naxos)
Day 12: Mykonos Town (windmills / Little Venice) is perfect.
Day 13: Beach day works — just pick one beach and enjoy it.
Day 14: Ferry to Naxos + Naxos Town + Portara.
Day 15: Either a half-day tour (villages/temples) or a hike — choose one.
Also: Naxos is famous for its long white beaches, so if Mykonos ends up more of a “party day,” you can do the quiet beach day in Naxos instead. Or skip beach day in mykonos add it here
Days 16–20 (Santorini)
You don’t really need 4 full days in Santorini unless you have the time (and budget)
Day 16: Ferry in + caldera views, easy day.
Day 17: Catamaran cruise (great choice).
Day 18: Oia or Fira (don’t rush both) depending where you stay.
Day 19: Relax + wine tour is a great idea — or an off-the-beaten-path tour with wine tasting.
If you’re tighter on time: keep Day 17, then on Day 18 do a private tour to cover the whole island (Santorini is small, it works), and fly out Day 19.
One important check: Santorini → Thessaloniki flights aren’t always daily, so confirm that early (or be ready to connect via Athens).

Adding more context, Thessaloniki because me and my wife are dropping off our son for a 3 week intern/study abroad opportunity. He won't be with us until the return flight. We'll be arriving May 2nd. My personal preference for a trip is food, culture, history. Beaches are nice to see and maybe dip our toes or a long stroll for the view, seaside dining, but we are not likely to spend a lot of time sunning on the beach. Maybe we drop some of the Santorini days for more flexibility elsewhere. Since we will be in Thessaloniki, what would be a less "disastrous" experience there?

I don't mind a car rental and figured we would at least rent from Thessaloniki to Athens.

Some general advice (concerning island travel) and as you mentioned you are into culture:
If you are on Mykonos, you should visit Delos.
You could also skip Mykonos entirely and visit Delos from Naxos. This would give you more flexibility to enjoy Naxos.
For days 12-21: Make sure you have 1-2 days to spare in between, that are not completely booked. It is usually advised to return to the mainland at least one day before departure. This is because it is not uncommon, that there could be ferry cancellations and you do not want your whole plan to collapse because you arrive a day later on e.g. Naxos.
Santorini wine tastings can be fit "in between" other points of interest, as they are spread among the island. As you are travelling in May you are able to keep those visits rather spontaneous.