Hey, I went to Santorini with my girlfriend in May/June 2007 and loved it, its a beautiful Island and provides tonnes of opportunities for the archetypal greek photograph - Blue roofs, white buildings and clear blue skies.
We went for two weeks and I have to admit did do quite a bit of lounging around because it was bloody hot! I'm not the lying around sort either and didn't get bored at any point. To be fair, its a volcanic island so the beaches are black and quite gravelly anyway - not exactly welcoming to lie around on.
So, what was there to do - well, an abundance of orthodox churches and little villages to go and visit, the village of Oia is a beautiful place, albeit a bit expensive and seeing a sunset there is an absolute must, but get there early because it gets crowded. You can pay for an excursion whereby you can see this from the deck of a yacht, we didn't do that though. This would be the most photographed sunset stylgeo is talking about.
You can go and visit the active bit of the volcano in the middle of the caldera (Nea Kameni), interesting - lots of vents and sulphur in the air and almost no cover so take plenty of water and sunscreen. As part of that excursion they let you go and swim in the warm water pools created by the volcano.
We stayed in kamari, on the east of the Island not far from the airport (to be fair, NOTHING is far from the airport on such a small Island! We also visited ancient Fira (or Thira) which is basically a bunc of rocks at the top of a mountain, although you can still make out where the buildings were. Worth it for the view of the Island. If you go to Fira proper, lock up your credit card, the place is known for its jewellery and some of it is exquisite, might be quite pricey now that the exchange rate has plummeted though. I had to restrain the girlfriend on several occasions.
There are other things too - the red beach, which is odd to be sure and some excavations of a town that was preserved when the volcan went of proper big style - apparently reminiscent of Pompeii but without the people. It was closed when we were there.
Bottom line is that we loved it, its no party capital and you may find that you do run out of things to do over two weeks if you're not prepared to lounge at all but we loved the Island, the people are fantastic and its on our list to go again. From a photography point of view, I would suggestv taking your kit. You'll regret it if you don't, loads of opportunity there.
Wow, rambled a bit there! This is by no means complete, but I hope it gives you a flavour of what we experienced. We rarely re-visit a place becasue there are so many other we want to see, but Santorini is on our small but exlusive list which breaks that rule.
Happy to answer any specific questions too (with less rambling, promise!)
