Santorini.

White Dress: Topshop, Jabong.com
Blue stripe dress: Tailor made from a skirt from Forever21
Sandals: London Rebel UK, Jabong.com

When to go: Months of July, August, September. Good sun, good winds, not the peak of tourist season. Spend at least 4 days, 3 nights.
How to reach: Land straight at Thira airport or seaport.

How to go about on the island: Rent a car. Preferably before you reach so that the car is there at the seaport/ airport. There are many cars and scooter shops in and around Fira centre but last minute availability might be an issue.

What all to see: Oia (Preferable to just spend the entire day from sunrise to during sunset and after sunset there :P. Again and again), Amoudi Port at Oia, Megalochori (beautiful, quaint streets, shops and restaurants), Imerovigli (a more spohisticated version of Oia), Akrotiri (Red beach nearby, ruins, Light house etc.), Fira centre (hustle bustle of tourists and street artists).

Where to eat: Sea food at Amoudi Port. Many many restaurants in Oia. India food at Jaipur Palace near Fira. Economical Chinese and Doner Kebabs at Fira Centre (cheaper than even McDonalds)
What to wear: August is very sunny but windy. White helps you keep cool in the heat and it looks amazing in pictures. Blue is good too. Go for flowy clothes, e.g., Kaftans, maxis, bell sleeve tops with shorts. Don't wear flowy short dresses as they can be uncomfortable in the winds. Jazz up your outfit with strappy sandals (make sure they are comfortable as you have to walk and go up and down steps a lot in Santorini). Long, tassle colorful earrings are a good-to-have to jazz up white outfits!