The eastern side of Santorini has nice wide sandy beaches, the sand is black (crushed up volcanic rubble) and f#*ken hot when the suns shining.
No wind when i was there but its kiteable, no where to hire gear as far as i know.
As gigantas said naxos has a school and nice long white sandy beaches. I had one session there, crap wind (bit light for 12m) but was glad to get out after lugging my gear for weeks through europe.
If you want to kite in Europe, Head for Spain/Portugal, plenty of sandy beaches and heaps of wind.