Ah dammit, we like going away around April and I thought it would be OK at that time of year. Though was thinking primarily of maybe Athens as I don't know too much about the islands - can't go anywhere that would need us to rent a car to get around, which is annoying as I;ve always fancied the Azores as well.
Athens is open all year. George, who ran the bar we used to hang in was from Athens and he'd leave Zante around October and winter in Athens then open up in late May/early June.
The Canaries has gone mentally popular in winter and has gone expensive, we've booked 12 months in advance and got a decent price for next Feb, same with Crete, we've booked August 2025 already.
Only came back from Crete last weekend, everything was open - in fact it was nice as it wasn't too busy with tourists.
The Water Parks are still shut aren't they (if that's your thing) I know the one we go to, Acqua Plus, doesn't open until mid May and I remember the taxi driver we used telling us it went quiet in winter. We stay in Hersonnisos and the hotel we stay at closes for the winter, I was chatting to the staff, one goes back to Holland for the Winter and one of the bar staff said she lives off her savings from her earnings through the winter. I think its just a case of doing your research and making sure the area you are in is open.