First of all, I would suggest the Cyclades. Mainly for the “meltemi” wind which keeps things cool. Naxos is a good idea if you're looking for a large island with lots of things to do. Plus it has Apeiranthos, which is a traditional (and beautiful) mountain village and is pretty cool in August. However, Naxos is also usually packed in August. It does have great food, customs and beaches, however. And maybe because it's big, the crowds don't feel so ominous.   For the solo traveler, I would suggest Donousa. It's a tiny island with great beaches and hiking paths. My personal favorite. Small, one supermarket, decent weather, not too hot due to the meltemi, and can't really get very crowded. Folegandros is also a good bet, but beaches are wild, not sandy. Plenty of wind though.   Lastly, I would also recommend Lesvos or Chios, a bit cooler than the other islands or Crete of course, mainly because it is so big you can find everything, peace and quiet on the the north side of the island. Avoid the Ionian isles - although I absolutely love these islands, they are too humid and very hot in August.