Any tips on clothing for RS Athens & Heart of Greece tour in May?

Shoes with good traction/soles would be my top recommendation. We were there in May , and while it was warm, it wasn't blazing hot. I took a pair of tennis shoes and a pair of Merrill Mary Janes (which they don't make anymore). One lady had a pair of leather sandals that sent her skidding on one of the walks. The climb to the Acropolis wasn't any problem, but the stones at the top can be slippery. I needed a jacket and scarf when we were up in the mountains. Some people brought swim suits, but Hydra was the only time it was warm enough during our tour and I just waded. Weather can be so unpredictable from one tour to the next, it's hard to give hard and fast advice.

We loved this tour.

Thank you, Patty! All very helpful advice. Duh-I hadn't considered cooler weather at the higher altitudes, so I'll definitely take enough layers in case it's cold. My walking Mary Janes served me well on the South Italy RS tour, so they will go in the suitcase along with my swimsuit, which I hope that I'll be able to use at least in Hydra. Your suggestions are appreciated, and I wish your happy trails in the future!

You will love, love, love this tour! There are lots of stairs and hills, and yes, they can be slippery when wet. So make sure your soles have excellent grip. No need for skirts, in my opinion, unless you often wear them. We just returned from this tour on April 9 so of course experienced cooler temps than you will, but yes, the mountains will definitely be cooler than Athens and the coast line.

Thank you, Luv2Travel. I am beside myself with excitement about experiencing Greece, and feedback like your makes it even more exciting! I'm glad that you enjoyed Greece. Your suggestions and comments are very much appreciated. Thanks for sharing!
Happy trails!