Greece – Northern Sporades Archipelago

I’ve done it. If air travel isn’t an option, then you’ll need to take a ferry from Skiathos to Volos. I think it’s a couple of hours, or less if it’s in high season. Then there’s a direct, but long, rail service to Athens. You mentioned Agios Konstantinos – that’s fine if you want to cut your land journey time and you have a car (but you’ll spend more time at sea getting down there). I also don’t recall a train station at Agios Konstantinos, so if you get there, you’d need to get a bus to Athens.

Hope this helps! I’d go for the ferry/train option again, unless you have a car in tow, in which case the second option will do fine. The drive from Agios Konstantinos to Athens isn’t that long…