Get guide books, go to non-commercial websites about Greece, examine a map, look for means of transport..
After that you will realize that your program is unrealistic. In 18/19 days you will never have time to see all the places on your list.
Why do you want to go Hydra? This is a place that could be recommended for people who stay several days in Athens and want to spend a day or two on an island.
If you go to other islands you don't need to add Hydra to your list.
Moreover if you go to Hydra you will have to come back to Athens before to go to another islands because Hydra is an island in the Saronic Gulf which has no ferry connections with the Cyclades islands.
Crete requires at least 5 or 6 full days to see a small part of it.
Either you stay there for about a week, or you take it off your list. (or plan an extra week of vacation)
Santorini and Mykonos are the two best known, most expensive and least authentic islands of the Cyclades and they have been transformed into theme parks for cruise passengers.
If you insist, stay in Santorini for 2 or 3 days to follow the crowds, but eliminate Mykonos. Unless you want to party every night with the world jet set in Mykonos, in the Cyclades you have almost 20 other much more authentic islands with postcard landscapes, white houses, blue domes, turquoise sea and superb beaches without the crowds and half the price.
Nafplio, Olympia, Kardamyli and the Mani Peninsula: Without a car you will be dependent on train and bus schedules and you will not necessarily have them every day at the time that suits you.
In less than one week and starting from Athens, I don't see how you can organize this kind of trip without spending 50% of your vacation on public transport and much of the rest to unpack and pack in order to change accommodation and wait for the next available train/bus