If you're looking for the easiest way, it's a taxi. A flat rate of €40 to the center of Athens (€55 from midnight to 5 a.m.)
Akadimias Street is quite long; if your hotel is located, say, between numbers 1 and 30, it's easily walkable from Syntagma Square.
Syntagma Square is precisely where you arrive by metro or bus X95 from the airport.
The X95 bus is another way to avoid getting lost and is also the cheapest (€5.50). You board the bus and it ends its journey at Syntagma Square. There are buses approximately every 15 to 30 minutes, depending on the time of day. You can pay on the bus with a Visa or Mastercard credit card or buy a ticket at the ticket counter in front of the bus stops.
The taxi journey takes about 50 minutes depending on traffic, the bus takes about an hour.
To get to the cruise terminal, there's no other quick and easy way than a taxi. Allow about 25 minutes depending on traffic. As Derek mentioned, be sure to specify the cruise port, the ferry port isn't exactly in the same location.
I second Derek's recommendation: if you're not familiar with Greek taxis, which tend to overcharge tourists, pre-book a taxi with Welcome Pickups. It costs €2 or €3 more, and the driver will wait for you at the airport with a sign with your name on it (or will pick you up at your hotel to go to the port).
https://www.welcomepickups.com/athens/
Also, avoid booking a taxi through your hotel, it could be even more expensive than with a taxi driver who likes to rip off tourists.
