This forum is so helpful so I wanted to share a few tips from our trip! This might not be anything new, but in case anyone is heading to the same areas...we were there from May 25th through the first week of June.
NAFPLIO - what an enchanting little city! It took two hours to drive from the Athens airport and most of the drive is on a nice toll road (you can pay tolls by card). Rental car pickup is across the street from the airport (but grab your keys from the desk inside the airport). Palamidi fortess was a highlight. The path was in the shade at 8am. Entrance fee is now 20 euros, but our teens were free. yiayia brunch was our favorite breakfast place of the trip. Lovely people, lovely food, beautiful scenery, and many activities in the area. I would return here in a heartbeat.
MILOS - two months before our trip, most flights were sold out so we had a very early departure. Plan ahead if you want to fly. Highly recommend the flight as it's about 30 minutes. Milos is stunningly beautiful, but to see everything you will drive 20-25 minutes to most places on very narrow roads where people go fast and don't always yield. Sarakiniko and Kleftiko are two of the prettiest places I've ever seen and late May was not crowded. However, it was cold and windy (high 60s and 30-40mph wind) when we were there so not good for swimming (we live in Colorado for reference). There are so many things to see and do that the weather didn't matter, except we had booked a private boat in advance - definitely wait until you are there to feel the weather and then book a boat! The north side is windier, but the south side is still very windy (the ocean is just calmer in the south). Tsigrado was the most protected beach we went to, but it's very small. If I were there in warm weather I would rent a kayak from Milos watersports and explore the caves at Tsigrado that way. Paleochori was a pretty beach on the south side with nice tavernas. Plaka is a wonderful town with really great shopping (ceramica kymbe was a favorite) and we were glad we bought items here instead of shopping in Athens. O! Hamos! is a fantastic restaurant where we had the best lamb I've ever tasted (cooked in parchment paper with feta). While Milos was beautiful it was not our favorite stop because of the driving (we stayed in Pollonia but ended up mostly away from Pollonia) and the overall vibe of the island – lots of people posing for selfies. I don’t have social media, so I was unaware beforehand.
SIFNOS – Tied with Nafplio for our favorite spot. We took the slow Aegean Sea Lines boat from Milos and it was a very nice ferry. We stayed in Apollonia and could walk via a path to the center with restaurants and shops. The food here is incredible and Kelari was probably our best dinner of the trip. A few places were not open for the season in the first week of June (everything in Milos had been open). The village of Kastro and walk to the Church of the Seven Martyrs is idyllic and what we all think of before going to Greece. The wind died down while were here so we swam at Chrysopigi and Faros (I preferred the scenery at Faros, but you can pick a fresh fish to eat at the taverna in Chrysopigi). Both are very relaxed beaches with people sitting on towels in the sand. Platis Gialos has some fancier tavernas (we had yummy sashimi at Yalos) and chairs to rent. There are many other beaches we didn’t go to. There are lovely ceramic stores in Platis Gialos and in Kamares. I was very happy that I schlepped a few pieces home. The vibe on Sifnos was very relaxed. The roads are similar to Milos, but people are not rushing as much, so the driving was less stressful.
All of us would happily return to Greece. For us, it was worth it to be there early season and be a little cold but not have any crowds. The people were incredibly friendly and it's one of the most beautiful places I've been.