Most Greece lists get written for July and then get recycled for every other month, which is lazy and wrong. By September the meltemi has usually eased, the sea is at its warmest after months of absorbing summer heat, and the crowds that make Oia or the Palace of Knossos unbearable in August have thinned by half. Ferry schedules are still at full summer frequency — Blue Star, SeaJets, and the smaller Dodecanese lines haven't started cutting routes yet — so you get the logistics of peak season without the body count.
That window isn't the same length everywhere, and that's the organizing fact behind this collection. The south — Crete, Rhodes — holds its warmth and its ferry service well into October. The Cyclades, Naxos and Santorini included, cool faster and get windier as the month goes on. Athens doesn't care about any of this; it's a city, not a beach, and September just means better walking weather and restaurant terraces that aren't swamps of humidity.
So this isn't five great places thrown together because they're all "good in September." It's five places chosen because September is specifically when each one shows its best version of itself, and ordered so that version builds into a coherent trip rather than a scattered checklist. A traveler with three weeks can do all five. A traveler with ten days needs to know what to cut and why — and that's addressed directly in each entry, not left for you to guess.
The traveler this serves is someone doing a real Greece trip, not a resort week. You want ruins and swimming, you want at least one island with actual mountain villages and not just a harbor and a hotel strip, and you're willing to take a ferry or a short flight to get it. If you want a single all-inclusive beach for seven days, none of this applies to you, and that's fine — this isn't that list.