We are planning on taking a Rick Steves tour in April 2016 that ends in Kusadasi. How can we get from there to Athens to spend 1-2 days in Athens and then return to the US from Athens?
Easiest would be to travel to Izmir Airport with your tour bus (leaving early on the last morning of the tour) and fly from Izmir to Athens on Agean/Olympic Airways. Traveling by ferry takes much longer, with at least one connection at Samos island. See also the Itinerary and Pre Tour info for this route: http://www.ricksteves.com/tours/greece-turkey/turkey.
We just finished the same tour in May with the same question. Most of the flights leave Izmir very early in the morning, but once a week there is an afternoon flight on Olympus Airlines. We stayed an extra day at Kusadasi, then took a shuttle to the airport for the afternoon flight. It worked for us.
You can travel by ferry via Samos or by Chios - Cesme - Izmir.
Samos has a twice daily ferry to Kusadasi which is near Ephesus.