With just 8 days I would stay in Athens for 2-3 days then the rest of the time entirely in Nafplio.
Nafplio is one of Greece's prettiest towns and perfect as a base to explore nearby World Class Archeological/Historic sites and so much more.
There's plenty to do in and around Nafplio to keep you occupied.
To go to Hydra you would either have to get to one of the ports from Nafplio like Ermioni or Porto Helli to go to Hydra then ferry to Athens.
If you want to see Hydra there is a day cruise from Tolo (near Nafplio) that goes to Hydra/Spetses.
I would leave Athens at the end of your Trip.
Here's the best website for Nafplio filled with all kinds of things to see, do and experience: