We have been to Santorini twice (and a third time coming up in September). I would suggest getting your own cable car ticket and meeting your guide at the top for an island tour for a full day tour. That is what we did on our past two trips and it worked out perfectly. We saw everything! On our second trip there were six of us and our guide made a reservation at a wonderful, much, much less touristy restaurant. Guides are expensive but in our opinion worth it, especially here!