We are flying to Athens 4 days before leaving on a cruise in September 2016.. Any suggestions as to what we should see and where we should go? We are staying near Pireus.
Not sure what the ports are you will visit, but here are some thoughts. In Athens (very close to Pireus) one of the often overlooked places to go is the ancient Agora which has one of the best preserved temples in Greece. It is within easy walking distance of the Parthenon that almost everyone visits. Syntagma Square is also interesting as is the Plaka area. If you go to Mykonos a short boat trip out to the island of Delos is well worth doing. If you visit the port of Istanbul, Ephesus is well worth the time and effort. If go to Naples and you have a full day, you can do Pompeii in the morning and Herculaneum in the afternoon. Or you could do tour of the Amalfi Coast. Rick Steves has a guidebook on the Mediterranean ports you might want to get.
"Rick Steves has a guidebook on the Mediterranean ports you might want to get."
You should start by getting this book, since it not only covers Athens, but likely many of your other ports as well.
And in addition to any replies you get here, look at Cruise Critic. Here's their Athens Port Review: (scroll down and click "Things To Do"): http://www.cruisecritic.com/ports/newport.cfm?ID=110
And here is their Greece Ports Forum: http://tinyurl.com/ax6ys4a
The Piraeus Archaeological Museum is surprisingly good and little visited. I particularly like the bronze statues.
If you are there on a Sunday the street market behind the metro station is lively.
In Athens itself I second the suggestion of the Agora. I would also recommend a walk up Filopappou, the next hill across from the Acropolis. It has excellent views back at the temples. We've written up some directions here. It's the second walk down.
We also really like the Keramicos, which has some great tombs and again good views of the Acropolis.
All of these are easily reached from Thisseo or Monastiraki metro stations which are about 20 minutes from Piraeus metro station.
You say you are staying near Piraeus. If this means you are on the coast you may find it easier to take the tram into Syntagma and start from there.
All depends on what you are interested: Ancient sites, shopping, partying, more local than tourist areas, etc. One excellent web site for Athens info and in fact all of Greece is Matt's Greek Guide