We have visited Greece three times, and have taken cruises between the islands. Also been over to Turkey a couple of times and visited Sicily and Malta. The easiest and best way to see Greece is by cruises if you are unwilling to rent a car.
We once came up the Adriatic and visited Croatia and Montenegro and ended up south of Venice. The east side of the Adriatic is difficult to travel on land as the roads do not really go from Greece to Slovenia. We still want to visit Slovenia, and return to Croatia to see the countries longer.
On land, we like to travel in straight lines to cities that are in the same general vicinity. We especially like Budapest, Bratislava, Vienna and Vienna--all easily reached by trains. We adore Dresden and the Saxony region of Germany. Another good trip is from Vienna to Salzburg to Munich and down into the Austrian Alps around Innsbruck.
We have driven from Munich over to Stuttgart and up the Rhine River to Bacharach, Koblenz, Cologne and ended up in Amsterdam. They have trains running this route too.