Packing for Greece in mid-late September

I always check the forecast where I'll be heading within ten days of departure and adjust accordingly. Weather does not always do what we expect it to. But, sure, in general Greece is hot in September!

It depends where you go in Greece but I tend to agree with your friends.

However, a waterproof, windproof K-Way style pocket raincoat can be useful, as well as a light and warm "North Face" style sweatshirt for evenings and nights which can be chilly. This is the kind of accessory I used about 1 day out of 15 in Greece (and I never go there in the summer)

September is still warm so your friends suggestion is fine. If you run into some bad weather you can always buy something locally.
Bringing too much in your bags can be a real pain as many places have stairs and carrying the bag up and down. In order to lighten the load when we travel to Greece we always bring clothes that you can mix and match for a different look. Someone once suggested that the most efficient way to travel to Greece is to have one day of clothing in the wash, one set of clothes for tomorrow and another set for the following day. We have been able to travel with cabin baggage since we started that routine.

Great comments. I'll add to it. Apparently locals don't wear shorts in the evenings, so I'll add lightweight pants.

I'd also check the 10 day forecast. We were in Greece last September into October. Santorini, Naxos, Nafplio, Monemvasia, Athens, so generally South. Weather was lovely, warm/hot during the day and slightly cool at night. No rain. Be ready for the Meltemi winds!! They blew for almost 2 weeks straight. I loved it!! It was a beautiful time to be in Greece!!!

Some great advice up thread. We were fortunate with lovely weather in the month of September 2016. I agree with stanbr to pack for warm weather and just what you need. I brought light weight Cotten/linen blend tops, and also bought a gauze top there. Skirts/skorts, dresses, shorts, crop pants are great. We were in the Cyclades the end of September and a light wt sweater was needed in the evening. Enjoy!

I've been once before at this time and will be going again 19-30 September. I'll check the weather but it's usually still quite warm. But if you're on the sea there's always that breeze/wind and it can get chilly at night so a light sweater and more importantly a light scarf will be on my list. I'll check weather before I decide to bring my rain shell or not. One year in early October an early storm had rolled in so we did get a bit of rain and highs were only in the mid 70s but we were told that was unusual.

We spent the first three weeks of September in Greece in 2018 in the Cyclades, Nafplio and Athens and I only wore my sweater on the airplane. I have actually named my two favorite sundresses that I wore "my Greek dresses" as they were perfect as casual and comfortable and all I really needed besides my swimsuit for the perfect sea.

While the locals are not inclined to wear shorts all the tourists do and there is no problem unless you are going to a high end taverna. I have worn shorts over 15 visits to Greece without a problem.