Hiring a private tour for visiting Meteora is very easy and the way to go if you want your own pace and also be flexible. Plenty of options, I am afraid I cannot recommend anything in public out of respect to the Terms and conditions of the website.
In general, Greece is not famous at all for their approach in accessibility. Crete is a big island and you will need to get a rental car in order to move around. The cities are in general flat (Heraklion, Chania, Rethymno) but again, if you want to make it to the beach, you will need to walk from the parking lot etc.
Even in Santorini, if you stay in Fira and you want to go to the beach in Perissa (say from west to east), you will need to drive there (taxi, private car or uber) but its super close from the Main Street to the beach.
In general terms, if you manage to solve the transportation thing (I assume that taking public transportation would be harder but having a private car would be extremely convenient in order to drop you at the front step of any place you want to visit), then most of the places are accessible like beaches, museums etc.
By the way, in Acropolis, there is a lift for disabled people or people with mobility issues. Ferries also have elevators or escalators. Most of the hotels also have elevators. But on the other hand, do not expect a toilet for people on a wheelchair in restaurants (you did not mention anything about wheel chair, I am trying to give you a perspective).
I am sure it will be a culture shock in terms of accessibility but in the vast majority, you will be OK (even if Greece needs to try a lot to satisfy people with accessibility issues).
Hope it helps.
