I had been to Athens before, however it was new for my husband. We took the super cheap metro into the city center (note, it is about 15-20 minutes of a walk from the port, but worth it). We only paid 4 euros for an unlimited day pass. We got to the Acropolis at 9:00am and walked through the ruins. After that, we were pretty tired so luckily we saw a segway tour. We ended up taking an hour and a half tour to the National Gardens, Temple of Zeus, Parliament building, and the old Olympic stadium. We loved the segway tour! I was also able to talk them down to 30 euros a person (plus the office had free wifi and gave us bottled water after the tour). I absolutely recommend the segway tour once seeing the Acropolis. We took the metro back to the port around 2:00pm (we docked at 7:00am).
After Athens our last port of call was Argostoli (not even listed on cruise critic). It was cloudy so we did not purchase an excursion (this was the only port we likely would have purchased an excursion). We just got off the ship and walked around and ate lunch. Not really much to see in Argostoli unless you leave Argostoli.