We did the Viking cruise from Budapest to Amsterdam this past Spring, in June, 15 days.
The weather was perfect, and kids were not yet out of school, so "less" crowds.
I LOVED it.
Yes it is expensive, but it was GREAT..
We are able to walk, but not mountain hikers, the town tours were easy for us. We also went off on our own to see a few castles, the Schobrunn Summer Palace in Vienna, etc.
It was our first trip to Europe, so it was a nice icebreaker to the various cultures.
We had the cheapest room and found we spent most time up top where you can see both sides of the river (compared to people who sat on their balcony).
We did get great pricing, it was $4,900 each with free airfare and free gratiuities (which is worth ~$250 Euros). We booked it about 5 months before the sailing, just because they kept sending books and the price was the same as last year but now had free air, so seemed like a deal.