The Ringstrasse is the part of Vienna I like most for its architecture and I don't know a city in Europe showing such an ensemble of beautiful buildings.
The Ringstrasse is not one single street; it is a succession of large avenues with monumental buildings, parks and luxury. It starts in the north with the Votivkirche, University and Rathaus at the Dr. Karl Lueger Ring, continuous at the Dr. K. Renner ring with the parliament. Architecture is all neo-something: neo-gothic, neo-classic, neo-baroque.
Then comes the top with the Burgring with views on the Neue Burg and the two museums KHM and NHM at the Maria-Theresia platz. Let's continue with the Opernring and the Opera to end at the Park ring. You can do the walk in day-time but also in the evening (Wien is a rather safe town).