Having lived on the neighbouring island of Ios, there is no way in hell I'd go to Santorini in August! Everyone, and I mean, everyone is holidaying in Greece in August; French, Italians, some Germans, and as Santorini is a tourist trap it will be heaving with people. Good luck finding a spot to watch the sunset. Good luck getting into a good taverna. The beaches will be busy - and as someone else said, it will be so hot!
If you want a great Greece holiday, head to the Cyclades' smaller islands. Or better still, hire a yacht and explore.
But no, Fanny wants to go to one of the most popular islands (though I bet Sir Snorts had wished it was Mykonos with its gay bars!).