First time in Greece and lost in my planning

Santorini is not known for its beaches. Look at Paros and Naxos instead.

That's helpful. Thanks!!

My choice would be two nights in Santorini for the iconic views and then on to Naxos for the beach. I too do not do well on boats because of motion sickness, however the fast ferry from Santorini to Naxos was just fine. Absolutely no problem on it. If your children like to be outdoors/hiking I can recommend a guide on Naxos.

I agree. When you arrive Athens take the next available flight to Santorini. You need about 2 hours to make a connection. While Santorini is not really a beach vacation it is spectacular and you will enjoy the views and the caldera sunsets. Santorini can be done in 2 days 3 is better. However since you have such a short time frame do 2 in Santorini then head to Naxos.

Take the fast ferry from Santorini to Naxos. Its a two hour trip on a very large ferry so it should be quite stable. Naxos is a perfect companion to Santorini. It has a lovely main town with a maze of back alleys leading up to the Venetian Kastro the walled part of the town. Lots to see and do.
I would look for a hotel at St George beach Agios Georgios. Its part of Naxos town and is a good sand beach that turns into small dunes past the built up part of the beach. Lots of hotels in all price ranges. It is just a 10 minute stroll along the headland to the main waterfront promenade of Naxos town. find a bar to watch the sunset over the portera the remains of a 6th century BC monument. You get all the activities of town plus as much beach as you want staying at St George. Hotel Kymata has a good reputation is well priced and just 100 meters from the beach. All of the rooms on the top 2 floors have 180 degree sea views all the way to Naxos ferry port. The top floor has 2 units with a second bedroom and a shared deck.
Naxos can be done pretty well in 5 days.

There are two airlines that service Naxos to Athens. Aegean/Olympic and Sky Express. both have 3 flights a day so you can be in Athens around 10 AM for your 2 days there.

You get all the activities of town plus as much beach as you want.
Images taken this May
Around Naxos town 2022 https://www.flickr.com/photos/stanbr54/albums/72177720299959098
Back to Naxos 2022 https://www.flickr.com/photos/stanbr54/albums/72177720299940917

I agree with everyone who says Santorini and Naxos make a great pairing. We did exactly just that last May, in addition to a few other destinations. We spent 3 nights on Santorini and 4 nights on Naxos. If you have time for 3 nights on Santorini, you could rent a car for a day and explore the island, such as the Akrotiri Archaeological site, Akrotiri lighthouse, red sand beach, and black sand beach, where your daughters can swim.

We took the very large and very stable Blue Star Ferry from Santorini to Naxos. It's a 2-hour ride. I am prone to seasickness, too, and I did not have any problems on the ferry.

In case you are interested, we stayed at Blue Dolphins Apartments on Santorini, and Hotel Palatia located at the beginning of Ag. Georgios Beach, an excellent location.

Thank you so much for your responses. Such great and specific advice. I'm swapping my focus to Naxos now and will book the fast ferry from Santorini. Loved the pictures too!

stanbr & Kwoo are right on, about Naxos attractions .. I will only add suggestions/info:

• SANTORINI - having stayed there numerous times (because of "newbie" companions for whom it was a must-see), I've found the "goldilocks" area to stay for 2 nights -- FIROSTEPHANI. It's right on the cliffside edge, plenty of places with the Famous Caldera View, and on the north fringe of FIRA, away from the hubbub there, but an easy 15-20 minute stroll in rimside path to FIRA attractions - museum, shopping, nightlife; avoids car-rental. For 1st night sunset dinner, Cafe Remvi in Firostephani has stunning view, NO crowd, moderate prices. OIA - to avoid huge cruise mobs (weekdays sometimes 5 mega-ships dock; nightmare!), go EARLY. At 8am either bus (or with 4, a taxi) to OIA, enjoy strolls, take photos, & when throngs invade 10-10:30, flee via bus to Fira, explore elsewhere. If u long for a climb u can taxi to Ancient Thera (high high bluff on E coast) where u can see entire island.

• FERRY to NAXOS - Depends on how limited yr time is; if you want to reach Naxos by noon, take 10:30 fast-ferry... it has small open back deck where u can stand briefly for some view of cliffs on departure. If u have more days to spare, big Blue Star ferry w 2 large rear open decks, leaves 3:15 pm, arr 5:15 or so. TIP - there's a Race up for deck seats (unreserved) so make the kids sprint up the escalator & stairs to snag 4 chairs while parents stow large bags on shelves at main (vehicle)level. A lovely "mini-cruise".

• NAXOS LODGING - Ag GIorgios Beach (also known as St George) is ideal for May stay, populated but not crowded, so convenient to port attractions. Another good option, Hotel Palatia, seaview balconies on upper floors. nice breakfasts moderate prices. ALso check Studios Kalergis, all units w balconies on beach, PLUS u get free sunbeds. Ag. GIorgios is shallow at edge, u wade out about 100 yards to drop-off for good swims - a welcome feature in Early May because water gets warmer earlier ... outer beach like Plaza is deep & beautiful but BRR.

• NAXOS EXPLORING - To swoop around island to mountains, ruins, scenery etc, U can rent car (prob about 40-50E/day for sedan for 4 adults) --but you'll need some rental agent advice to plan itinerary & some "homework" to appreciate sites/towns. U can also do a good day-long bus tour (w. lunchtime swim-stop) for about E30 each - https://www.naxosbustransfer.com/tours/naxos-highlights/ As stanbr's photos show, Naxos Town itself is fascinating to explore.

Just a bit of additional information on the Ferry Santroini to Naxos. Jan is mentioning the upper deck outdoor space on Champion jet 1 and 2. Both have outdoor deck space at the rear of the craft. Probably enough room for 40 people up there. We timed the trip from Satnorini harbour to the outer rim of the caldera. It is in the Caldera for 15 minutes.

On our last visit in 2022 we were on Power jet. It has a much larger open deck at the back. It probably would hold about 100 people.

Love the information about the ferries (especially seating on the deck) and where to stay and how long in Santorini and Naxos. Our flight coming back to the US leaves Athens at 12:50 pm. Would you suggest flying back to Athens from Naxos that morning or leaving the night before? It looks like it's doable to just fly back that morning, but I realize that could possibly go awry. Would love to hear your thoughts and experiences.

Thank you, Thank you, Thank you for your advice!! Totally amazing!

Night before, night before. After a wonderful re-energizing stay in Greece, why add stress at the very end? If you were staying on the mainland, a 1-2 hour drive from the Airport, maybe same day --- but not where air/sea disruptions could occur!

Just to correct my ferry advice, there is no seating on the open decks at the back of the ship its standing room only. And beware the ferries do 30 knots so its really windy. Most people only last 10 minutes or so before they return to their nice comfortable aircraft style seats inside.

On one of our trips we took a flight from Naxos to Athens. The flight was delayed for 3 hours (mechanical problems) Panicked Americans, Brits, French and South Africans who were planning same day connecting flights home all missed their flights. We had our return flight home booked for the next day. My little story, to reinforce Janet's advice. Be on the mainland the night before your return flight.
You do not even have to go back into Athens. If you can afford the Sofital it is 50 meters from the terminal building. You could also consider Avra hotel in Rafina. Its a nice hotel with sea views about a 20 minute commute from the airport. They provide a free 24 hour shuttle to and from the airport.

Would you suggest flying back to Athens from Naxos that morning, it
looks like it's possible to just fly back that morning

No, that's not possible.
There are no ferries that will arrive in Piraeus early enough for you to catch your 12:30 pm flight.

Moreover, all the regulars of ferry trips in Greece like janet, stanbr and me, will tell you to return to Athens at least 1 day before your return flight because you risk being stuck on an island like a few thousand tourists each year who have forgotten that ferries can break down or not sail at all due to bad weather conditions.

Personally after being twice, in the 80s & 90s, one of those tourists stuck on an island, I always come back at least 3 days before (yes, it happens that there is more than a day without any ferry).

So if you don't want to miss your flight and enjoy Athens, save Athens for the end of your trip.

Yikes! I was convinced when Janet said not to do it, but the stories of being stranded and stressed out have made a true believer out of me. I will take STANBR's advice about going to Santorini first. I think I will then go to Naxos and put Athens at the end. Again, such valuable advice. Thank you all!!

Svailsmith, yes, that's the best plan. That is what we did last May: flew to Santorini first, then ferry to Naxos, then flew to Athens where we picked up our rental car for Nafplio, and we ended with 4 nights in Athens.