Just a point on your proposed Samaria Gorge trip. Have you ever walked down the gorge?
I did it about 14 years ago when I was a lot fitter than I am now, and remember falling asleep on the ferry from Aghia Roumelli to Chora Sfakion. It's a very early start and a long day of walking, so I don't think people will be interested in a beach barbecue on the way back. What is needed on the way back is the quickest trip back to Plakias as possible.
Perhaps you could do the Samaria Gorge the easy way, i.e. take people by boat to Aghia Roumelli and bring them back. You would see Loutro, Chora Sfakion and Frangokastello on the way. I don't know long it would take, but it might be too far for a day trip, unless you had a fast boat.
Another suggestion: Some of the "far enders" might fancy a trip to Aghios Pavlos (between Preveli and Aghia Galini). You can get boats there from Aghia Galini but not, AFAIK, from Plakias.
Greecemad.