The Wachau is a scenic stretch of river valley in northeast Austria. It is basically Austria's version of the better known Rhine valley in Germany. It's an easy day trip from Vienna or Linz and is often combined with the spectacular abbey in Melk (unless you're toting around a two year old, in which case you are avoiding abbey tours).
The northeast of Austria is prime vineyard territory and the Wachau features steep terraced hillsides along most of the valley.
The Danube is the focal point through the Wachau. Touring the area by boat is quite common, there are stops in most of the small market towns along the river. There are river roads on both sides although the one on the east side often wanders off from the Danube for a few miles. There are no bridges between Krems and Melk, just a couple of car ferry options for crossing.
There is also the possibility of train-based touring but the train stations in several of the towns are a considerable walk from the ferry / boat docks.
Any hillside that isn't covered in vineyards is likely forested with a castle (ruined or otherwise) crowning it. This is Spitz castle. Most of them are open for wandering about. Durnstein's castle is supposedly where Richard the Lionhearted was held captive, so it seems to be a tour bus focal point.
Most visitors don't seem to stay in the Wachau, but the towns generally do have some small inns, and they all have lovely riverfront beer gardens. This one, in Spitz, also has a playground included. Most of these beer gardens also cater to cyclists touring the Wachau.