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.
We took RCI transportation into the city center and walked around. We also toured the old fort for a couple euros and walked around the outside of the new fort.
Must walk inside the city center and up on the city walls.
Our favorite port! We took the cable cars up (4 euros per person, per way) and walked around in Fira until we found the bus station. We took a bus (only 1.60 euros per person per way) from Fira to Oia. Simply board the bus and pay when you are on the bus (they do give change). The bus ride takes around a half hour. Oia was breath taking. Loved the little shops and the views were amazing. We took the bus back to Fira and then walked down the flights of stairs to get back to the ship (be careful - my husband slipped and broke a shot glass because the stairs are slippery).