Cotton is the best fabric for Greece as far as tops and socks are concerned. Denim and cotton the best for shorts and trousers. Light is the way to pack. Remember you have to lug this stuff around. Always bring half of what you think you need. A couple of shirts, tee's, shorts, slacks, one windbreaker and one sweatshirt should do. Sandals and flip-flops you can buy here.
The average summer Grecian temperature in July and August is 90 F or 33 C and plan for no shade. Summertime Greece has a dry and arid climate but its often windy on the Greek islands. Summer nights are surprisingly short; only about 8.5 hours long and even shorter if you are having fun or sleeping on a beach somewhere. After a night of island partying Greeks sleep late, chill as much as possible and only venture out when and if they decide to go to the beach where they may lay around from 1:30 to 8:30 PM. With the days so long there is no hurry to get to the beach.
To be prepared for all this sun I recommend a white cloth type of hat that you can fold up in your suit case or day pack and your pocket and even swim with. I also recommend and XXX-LG white Tee shirt you can wear on the beach when you start to feel the sun get too strong or even before. Many beaches in Greece have become "organized", that is, they rent beach umbrellas and loungers for 3 to 6 euro depending on the beach. By law there has to be an small area without these amenities where you can just put your towel down and sunbathe too. If you fall into the later category and eschew the umbrellas you'll need the above hat and tee shirt. The sun is nice but only is small doses. Some Sun screen even has carcinogens in it so shade is mandatory for many who are not used to the hot sun. There are no clouds and even if there were the ultraviolet waves still can cause sunburn.
The idea of these organized beaches is pretty new in Greece and really got me all steamed up before I mellowed out about them and started preferring the shade. I am surprised they didn't think of it thousands of years ago too? Maybe they did.
What really gets me steamed up though is when a beach side bar starts blaring really loud techno-music all over the beach. This happens all over Greece and there are laws against it too! Greek Laws and E.U. laws both. Its just another instance of friendly Greek police having strava matia and looking the other way accidentally on purpose as they bend over to pick up an envelope. So you may need to bring some more headwear:
In General for hot climates packing 100% cotton clothing is the best choice, with linen and hemp close behind! Because its so hot in Greece you will perspire and cotton wicks away moisture. Each has its advantages but cotton is the easiest to carry and care for of the 3. It dries quickly and Greece is a good place for quick hand washing. Sun and breezes!
Next : Avoid these common dangers of Greek beaches