Very easy to DIY. Quite confusing initially coming out of the terminal and Celebrity was useless. Once we had our bearings took the tram to see the main sights.
Acropolis was cool, otherwise city not that exciting. We did hop on hop off bus to Acropolis. I would be very careful with this, though. They advertise every 30 minute buses but can run every45, and we witnessed our bus filling up and leaving people behind on the return. This is probably a port best done with private tour