Rick Steves book was published before the X80 bus was instituted. Several reasons to reconsider:
• X80 starts right by the cruise ship pier. Metro is over one mile away... on this link, THe Cruise pier is the one beyond the red letter K -• http://www.mappery.com/maps/Pireas-Hotels-Map.jpg ... and believe me, that's a looong walk. by the time you get to the Metro, the x80 may very well be reaching the acropolis.
• The Green Metro does not go to Syntagma Square, it goes to Monastiraki, and you would have to change to another line to get to Syntagma square. Do you want as a "newbie" to negotiate a busy subway system you know nothing about?
• If you emerge from either station you are still a 30-minute walk from the Acropolis.... of course you say you want to do a bunch of other walks first, but I think that is unrealistic (see next point).
• I applaud your ambition at wanting to do it all. However If you start with everything else BEFORE you go up on The Rock, you very likely will find yourself having to skip the main event! Do Acropolis FIRST -- and you can skip one of the other things if need be.
• Getting back to Ship: Metro vs. X80: Remember, your ship will Pull up the Gangplank at 5 pm. Even if you take a Metro back to Piraeus it's 35 minutes on train, and remember you still have to count the time to get to Monastiraki, buy a ticket, get down to the platform, wait for train, disembark fight your way through the crowd to the street -- and then you STILL have a 30 minute walk to the Ship!! (and good luck getting a Taxi in Pireaus!!)
• If you take X80, that link I gave you shows exactly what it does -- it's 30 minutes to Acropolis & another 15 minutes to Syntagma. You could get off at Acropolis, go up top, go down to Ancient Agora, go to Acrop Museum and THEN, only then, if you have time, head for Syntagma. The driver will tell you where the stops are and WHEN. You should not really wait longer than 3:30 to start your journey back to the boat.
• Especially if it's a Free Sunday, you should try to get to the Acropolis first, because of crowds; it will peak about noon.
• And finally, the Metro is a Pickpocket target, the bus will be pickpocket-free because of limited stops.