Need suggestions for reliable trip planning help

Conde Nast Traveler and Travel & Leisure magazines both have lists of recommended travel agents, you might look through them.

Also depending on where you live, the Auto Club still has a robust travel department in many locations.

It appears your need is advice on what Island you visit between your two visits to Athens.
I would suggest you consider the city of Nafplio. Its a two hour commute from Athens on the mainland. You can get there by taxi or private driver or Bus. Its a lovely seaside city with an old town and topped with a Venetian era fortress.

We were lucky to be there last year during Easter.
There are a number of ancient sites nearby as well.
Here are a few images.
Nafplio https://www.flickr.com/photos/stanbr54/albums/72157632094108982
Nafplio 2024 https://www.flickr.com/photos/stanbr54/albums/72177720319228274

Thanks for the suggestion for Nafplio. This would solve the dilemma of spending a large part of two days getting to an island. My main concern is finding activities to interest our 14 year old, but it appears it will be too early for any real water activities then. What are the hiking opportunities like in Nafplio? Also, I read about a tiny island near Nafplio which I neglected to make a note of. Do you know it and what is it like? Thanks for taking the time to reply.

I suspect the small island at Nafplio is actually the Venetion era prison called Bortuzzi in the Nafplio harbour. There are small boats at the wharf that take you over to it but you can't get inside so you spend 20 minutes or so in the outside. Your 14 year old is going to like that little excursion. The huge fortress up in the mountain over Nafplio is a great place to visit. It can take 2 to 3 hours to visit.
You can see Bortuzzi in several of my photos.
You might also be interested in a hotel. We always stay at Agamenon. Its the closest hotel to the Sea and we have always been able to book a sea view room.

My young adult children loved Nemea which was a site of Greek games like Olympia. There are some reconstructed sites, an original track you can race on, and a very well done museum.

They also loved the castle in Nafplio-we spent a morning there including climbing the 999 steps-there are other ways to the top. Epidaurus was enjoyed by all. Mycanae had fewer fans as it requires more imagination.

Nafplio is a pleasant town and a place you can let your grandson wander around. My 19 year old son found a small store selling a little of everything he returned to several times and even went back to good bye to the proprietor when we left.

We enjoyed Naxos too but I would go for Nafplio in April.

Our Greek friends who live on Crete sent us to Maria st Diktynna-travel.gr. They helped us plan our stay in Loutro on the south shore.

If you decide on Nafplio you'll find more than enough to keep you busy for 3 nights and more.

You can take a bus from Athens to Nafplio (about 2 hours) or rent a car in Athens and drive yourself.

There are also private transfers from either the airport or Athens and may be more cost effective and convenient with the number of people in your party.

There are numerous third party websites to find accommodations and after that you can decide what you want to do in and around the area.

There may be some agency that can help you arrange the trip if you want to go that route but I'm not familiar with any.

Here's the best website for Nafplio and you'll find a lot to do in and nearby of the town:

https://visitnafplio.com/

Thanks to all who replied. Very helpful and I now am making progress on concrete plans. Three nights in Nafplio looks like a wonderful option for our schedule and interests.