There were taxi's waiting at the port during my cruise in June. You can also walk to the Metro station. It is about a 20 minute walk along the road that curves around the port area. The Metro station is in a non-descript yellow building. You can buy a ticket to the Acropolis area for about 60 cents (Euro) or about 75 cents if you expect to transfer to the M2 or M3 lines. The M1 line is the only line to Piraeus. We got off a Montersarki (or something like that) and walked through the Plaka area up to the Acropolis. It was inexpensive and easy to do. The train ride took about 15 minutes.