We did this on our own. Walked up the old town wall to the top for beautiful views and then came back and saw the old town. No need for a guided tour unless you absolutely have to know the history and significance of everything. Our pictures are at:
With fellow CruiseCritics Roll Call members, we did a Rick Steves-guided walk through the heart of Athens and then the Acropolis. Not having seen the Acropolis before, we were absolutely blown away by what the Ancients had accomplished. Pictures at:
Cruise company provided shuttles to the Castle of St Peter. However, the frequency was very low, about 3-4 shuttles each way all day! Not too long a walk, but it was HOT. The castle is interesting and has a wonderful collection of antiques recovered from shipwrecks.
Good tour but expensive - 800 euros for 14 people for a 5-hour tour. The vehicle had room for 22 so when we added two, the company asked for 100 euros more(!) but eventually relented and did not charge any extra. Santorini is beautiful. We rode up on the donkeys - enjoyed it. Went down on the funicular. Walked around the little town after the tour.
Six of us took a taxi to Krka National Park for a round-trip cost of 250 euros including the wait at the park. We spent about three hours at the park. After a few days of old towns and ruins, we thoroughly enjoyed the natural scenery and the amazing waterfalls.