We will be in Greece and wanted to pop into a Turkey if possible. I see there are buses or train from Athens to Istanbul, albeit pretty long. I assume you would just get your Turkish visa (for US citizen) and then take the train over?
Also interesting though, (according to Rome2Rio) there seems to be a ferry route from Athens to Ephesus. [Island hopping : Piraeus -Vathy, Samos - Kusadasis]. Can you just take the ferry over with your visa and get entrance to the country?
There might be some other island hopping routes too with so many Greek islands.