9 days in Greece

One option is to read RS' route through Athens and the Peloponnese and do some sort of mimic of that. Do you land on day 1 and depart on day 9? Thats a short stay. You will want 2-3 days in Athens (we stayed two and that was sufficient to see much of what we wanted). Head by car to Delphi ,Olympia and then Nafplio. This would be an abbreviated loop. You may be able to fit in Kardamyli, and then drive up over the mountains on the back of it to Mystras and then on to Nafplio, but that would be a fast trip. Maybe just Delphi (1), Olympia (1), Sparta/Mystras (en route to...) Nafplio 2-3, Athens 2-3. Thats a rough plan

I've never traveled to Greece in April but from all accounts should be a nice time. It won't be beach weather but you won't have the heat of summer to deal with.

It would help if you provide what your interests are: culture, history, food, shopping, partying, touristy, non-touristy experience, etc. Do you want to rush around and try to see as much as possible or take your time and have more quality than quantity? Remember, every time you go from Point A to Point B eats up time for travel as opposed to more time at your location.

April will offer lower prices for accommodations especially on islands.

You can combine the mainland with islands or just the mainland.

I would recommend staying in Athens for 2-3 days and explore the city and it's world-class archeological sites, museums and older parts of the city.

Then decide if you want to go to an island or islands or stay on the mainland.

If it was me I would combine Athens for 3 days (at the end of your trip if possible) with 1 or 2 islands that you can easily connect to via ferry.

Way to many options so a bit more research on your part will help.

Check out this wonderful website for Greece: https://www.greecetravel.com/

First decide how much time you want in Athens. Most would give it a few days, but I had other priorities and intend to return so I'm only spending a day there. You might feel like this will be your only Greek trip or otherwise have reasons/interests to allot a certain amount of time there. But I would certainly start by determining how much time you have available for places other than Athens. If you deduct 3 for example, then you know you have six to play with.
April is early for islands, so I would look at mainland.
My own trip in May will be a tour of the Peloponnese. Do you have a guide book? I know Greece is overwhelming to plan, but that is the best way to start.

As far as guidebook suggestions we found The Rough Guide to Greece and RS Greece Guidebook very helpful when planning our tour. We can't speak to April since we were there in the month of September. With 9 days, think "less is more" and choose a few spots to soak in the Greece experience.

Thank you so much for sharing your experiences. It helps a lot, wish you all a great weekend.