I loved santorini - Perissa and it's smaller neighbour of Perivolos were great - decent beaches (black volcanic sand). 15 mins by road (cab was about 8 euros) from the main town of Fira. The view of the Caldera (the blown out volcano) and sunsets at Oia (pronounced ear) are breathtaking.
I also really liked Parga on the mainland although the beaches are more shingle and the water colder as it's a natural inlet to the sea from the mountains
Agree with this I thought the caldera was stunning worth the slightly higher prices