Hoping for feedback from the forum's many Greek experts!!!

You’ve done a great job researching and planning your trip and you made an excellent choice with the islands. You definitely won't get the usual crowds found on other Cyclades islands (Santorini, Naxos, Paros), instead, you’ll enjoy authenticity, lower prices, and the same postcard views.

As you’ve learned, ferry schedules don’t change much from year to year but that doesn't mean you'll get the exact same schedule in 2027 as in 2026 (or 2025, which you can look up on openseas.gr).
Since you won’t have any issues booking ferry tickets in September, your best bet is to wait until the Sept2027 schedules are officially published, which usually happens a few months in advance.
This is especially true for getting to and from Kimolos and for your final Folegandros–Piraeus leg.

If there happens to be no direct ferry to or from Kimolos on your planned travel days, take one of the daily ferries running from Sifnos to Milos, once in Milos, take a taxi to the other port "Pollonia", from Pollonia, a small local ferry (Osia Methodia) runs to Kimolos roughly every hour. You can apply this exact same Plan B logic if you need to travel from Kimolos to Folegandros by transit through Milos.

The Osia Methodia Ferry: https://kimolos-link.gr/en/

When it comes to island hopping, flexibility is key. You're never completely safe from a delayed or canceled ferry and you just have to roll with it. As for accommodations, unless you've found a rare gem where you absolutely must stay, don't rush to book everything.

Even though you don't seem planning on renting cars, I usually recommend renting one on each island because it opens up a lot more accommodation choices. That said, a car won't really be necessary on Folegandros (at worst, you can rent scooters).

A car on Kimolos can be useful, not just for a wider selection of places to stay, but also to explore the remote beaches on the west coast. Some of them easily rival Milos's famous, heavily Instagrammed beaches (minus the crowds). There’s a minibus service linking the port to the main town (Chora), and another minibus that runs along the south and west coasts of Kimolos, though the schedules can be pretty unpredictable.

Either way, given how limited accommodations are on Folegandros and Kimolos, those are the places you should focus on booking first.

Sifnos has an excellent bus system with its main hub centered in Apollonia/Artemonas and I completely agree with your idea of spending an extra night in Sifnos. If you do that, I’d take a night off Folegandros, assuming the ferry schedules allow for it.

Does it seem like a good idea to spend the night in Piraeus before
heading to Nafplio the next morning?

Given your plan, on the day you take the Folegandros–Piraeus ferry back, I would spend the night in Piraeus and the next morning take a rental car and drive to Nafplio (the drive from Piraeus to Nafplio takes about 2 hours). You can then return the car in Athens when you come back from Nafplio. Plus, having a car will let you organize your sightseeing around Nafplio completely at your own pace.

How much time is generally needed between landing at ATH (we will have
carry on luggage only) and catching a ferry out of Piraeus?

Since no one can predict potential flight delays or wait times at immigration, I can only give you details for the trip from the airport to the port of Piraeus itself. It generally takes between 1 h and 1h20 depending on your mode of transportation. The main differences are the price... and a bit of walking.

Taxi: approx 1h / €60, no walking.

Metro: a train every 36 minutes, €9 ticket, 1h journey, some walking to reach the correct ferry gate (about 20 minutes to walk to Gate E9 if taking a Seajets ferry.

X96 bus: a bus roughly every 20 minutes, €5.50 ticket, 1h15 journey, plus about a 20-minute walk to Gate E9 .

To sum it up, don't rush. Keep doing your research, you've got plenty of time to fine-tune everything

Regarding Nafplio leg: I would echo JoLui in urging you to consider renting a car if you want to the sites around Nafplio. We have only rented from the airport but the roads really are not difficult to drive. A good portion is toll roads. We just got back from Greece and used Apple play in our rental car which simplified navigation immensely.

I have also read of people renting a car in Nafplio although I have not done it.

Getting to Nafplio by public transportation requires getting to the bus station. I have not done it but it is in Athens not near Piraeus or the airport.

Thank you so much for your replies, JoLui and Beth FL!

JoLui: I appreciate your feedback so much......it definitely gives me more confidence in my itinerary. Based on your response, a couple more questions and comments:

1. Once the ferry schedules for September 2027 come out, is it necessary/less expensive to book ferries in advance? I have read several say there is no reason to do so, especially in September, but is there a financial advantage to booking ahead of time or is it truly better to just show up and purchase tickets at the port?

2. I'm really glad to know there is an alternative route via Milos to and from Kimolos, especially since I wasn't delighted with the only ferry (as of yet) between Kimolos and Folegandros being so late in the evening. I also think it would be great to spend an extra night on Sifnos. You mentioned taking a night off Folegandros rather than Kimolos. Can you say why?

3. In terms of booking accommodation, as you mentioned, I definitely have lots of time. Putting Kimolos and Folegandros as the priority, how far ahead should all accommodation be booked, to get the most choice?

4. We were planning to rent cars on Kimolos and Folegandros at the least, as I understand Sifnos has a very good bus system. Is it still worth renting in Sifnos? Why not Folegandros? I believe I've read you say that it is best to rent locally from agencies that only require payment at pick up. I assume this is an option on the islands I've chosen and that I should research local agencies and contact them directly in the months before our trip?

JoLui and BethFL: I appreciate your thoughts on renting a car from Piraeus to Nafplio. It definitely crossed my mind. We would like to avoid driving in Athens and other large city centres so I will do some research on whether there are pick up and drop off locations that allow for same. I will also see whether the cost goes up considerably by returning the car to a different location.

Thanks again! I am sure I will have many more questions in the months to come and will reach out for more expertise.:)

JoLui almost said it all already, but I will try to add something as well:

1.) It might make sense to pre-book your first ferry departing from Piraeus to get a small early-booking-discount, but usually the cheap tickets are really limited (and to be honest I only know them from Blue Star ferries, who do not operate in the western cyclades). For all the other ferries, I would not pre-book it, to give you maximum flexibility for your island stays.

3.) Personally, I usually do not pre-book accomodations until maybe days or hours before, but from what I observed, I would say that on common booking platforms, most rooms are put online after End of March. Though, I would recommend to also check accomodations that are not on the big platforms and can only be contacted directly.

Your overall island selection is nice and also the time on each island is reasonable, giving you plenty of space to deal with possible ferry delays/cancellations (e.g. if ferries get cancelled to Folegandros or only have very inconvenient connections, you could just add a few days on Milos) or just staying somewhere longer if you like it.

1/ Like blackdog said (and not R Plant! ), you can book your first ferry trip from Piraeus to Sifnos in advance. This isn't really an availability issue, but it will save you from having to do it at the last minute, especially since you won't have much time to spare once you arrive in Athens.

There is no discount for buying your tickets in advance. Blue Star Ferries does offer a few deals but as blackdog mentioned, they don't service the specific ferry routes you'll be taking.

For all your other ferry rides, it’s completely up to you. You can just buy them on the island you're currently visiting before to head to the next one. Keep in mind that ferry schedules can sometimes change. If you booked tickets and the schedules are no longer valid, you will need to get your tickets updated by visiting a local ferry ticket office on the island you are on.

2/ The Sifnos-Kimolos route is operated in September by Magic Sea Ferries' "Cat 1," departing Sifnos at 1:50 PM and arriving in Kimolos at 2:30 PM. This isn't a late arrival, especially since you likely won't be able to check into your accommodation before 2:00 PM anyway.
But keep a backup plan via Milos in mind, especially for the next leg of your trip to Folegandros. The direct route to Folegandros is only operated on Mondays by the Dionisios Solomos. If you transit through Milos instead, you’ll have access to a daily ferry.

We stayed longer on Kimolos than on Folegandros. In our opinion, there was simply more to do on Kimolos, and it’s a much better island for exploring (we always like to do a few hikes). At the end of the day, the choice is yours. I do think Folegandros complicates your itinerary a bit, and without trying to change your mind, have you considered the island of Serifos? It’s located right between Piraeus and Sifnos, and I think it would really simplify your ferry routes.

3/ There are different approaches to booking accommodation in advance. You can either do it at the last minute like blackdog, which is more or less what we did for Folegandros, since it wasn't originally in our itinerary.
Personally, for a September trip, I start looking around early May.

I should mention that we are a bit of a special case because we have very specific criteria when picking our places (location, aesthetics, furniture, kitchen amenities, outdoor spaces, overall style) it’s as if we were planning to stay there for months! Also, we never stay in hotels.
Because we look for very specific rentals, it means we have to find the perfect spot and lock it down before anyone else does. As I mentioned before, Kimolos and Folegandros are small islands with limited options. I really wouldn't recommend the "last-minute" approach if you want to have a decent selection to choose from.

4/ Folegandros basically consists of a single road for about 10 miles from the port to the very end of the island, where your only option is to turn around and head back. The local bus runs this exact route every hour. It’s up to you to decide if you really feel a car is essential in this kind of context .

On the other hand, a car was useful for us on Kimolos. And not just for us, we actually picked up a few hitchhikers along the way, including an American guy from New Jersey who was lost in the middle of nowhere and mistakenly thought he could catch a bus. He literally called us his "rescue team."!
Back then (in 2021), we rented a car through RAC https://www.rentacar-kimolos.gr/ . The agent was waiting for us right at the Kimolos port with the car, and we returned it to the port 6 days later before catching our ferry.

I can give you a few names of rental companies on Sifnos since we’ve been there several times (the first time was back in the '90s), but they are easy to find on your own by searching "rent a car" on Google Maps.

While I'm at it... here are a few highlights from Kimolos.

https://photos.app.goo.gl/5yH9SmD5yeKc7jHL6

Thanks to blackdog and again, JoLui, for all your super helpful info! I will definitely book our first ferry out of Piraeus in advance and otherwise buy tickets as we go. I love to research accommodations so I know I will book those in advance. Like JoLui, I know what I want when I see it, so I wouldn't want to miss out by waiting. I will be looking for locally owned properties - small pensions or house/apartment rentals that are simple, inexpensive and have outside space, I have no interest in resorts or hotels with pools, etc.

The late ferry I was not thrilled with is the one from Kimolos to Folegandros, which arrives at 9:35 pm. Knowing I can avoid it by going through Milos makes good sense to me. But now my interest in Serifos is piqued, so I will definitely explore that option.

Thanks for the video, JoLui......looks amazing!

Just in case you're interested in Serifos:

https://serifos.gr/en/

It’s a pretty mountainous island with gorgeous views. Livadi is a lively port town with a nice beach nearby, and the capital is perched high up on the hills. On the island, you’ll see an interesting mix of classic, whitewashed Cycladic architecture and traditional stone houses.

Serifos is located right before Sifnos and is serviced by the same ferry routes, which could make things a lot simpler for your itinerary and schedules. (worth double-checking, though).

A little glimpse of Serifos:

https://photos.app.goo.gl/KxFRyPy6jFtNWSBa6

It shows the exact kind of accommodations we love to stay at.

Your videos are so helpful, and give as good a sense of a place as possible without actually being there, JoLui. Thanks very much....it will be helpful in making a decision, ultimately.

If you want a cheap(er) ferry to Sifnos, Kimolos and Folegandros, check if the Dionisios Solomos from Fast Ferries is there on your intended days of travel. The route of this ferry is a government subsidised route because there are stoops at some so-called "barren" islands. It also means that ticket prices are low.
https://fastferries.com.gr/en/routes/

Thanks, Nicksy, for that tip - will do!