That's a nice choice. We already have plans for this summer (Newfoundland, PEI, etc.), but are looking at a Viking tour doing Budapest to Prague for 2017.
The River Cruise was the best. No 2 days at sea to get somewhere, bring your room with you, stay on the boat if you want (we never did).
At the time Viking did not offer a cruise from Budapest to Prague, but we went to Prague too and had to take a 4 hr bus ride to get there. I need to look at a map and see if the river goes from Budapest to Prague.
Two days there to roam around down town and went to the Terezin Concentration Camp, near Prague, sobering.
In both Budapest and Prague interesting perspectives about being post USSR.
Budapest was the coolest city we visited on that (Amsterdam-Budapest) trip. However the coolest place (for me) was the
Windmill World Heritage site at Kinderjik, quiet at the time of year we went (end of Oct) peaceful, pastoral, and like stepping back in time.