In Athens, we actually did two tours during our free time. We used Trip Advisor to find a private tour company called Private Greece Tours, and they did a great job for us! We did a half day excursion to the Temple of Poseidon, and we thought it was amazing! Gorgeous setting and scenery. We had flown in one day prior to the start of the
ABD, so we did this tour during the day before our welcome dinner. We felt it was very worthwhile.
Next, we did a tour called "Athens by Night" where we got to watch the sunset high above the city, and got to visit some of the sites after dark to see them all lit up. It also included time for a dinner stop of our choosing (you pay for dinner separately). We ended up dining waterfront at a spot near the port of Piraeus that was recommended by our driver. It was a very nice evening!