It is a beautiful city in a "high-status" manner, with ornate huge buildings and large spaces. However, it is possible to see "everything" in the central part of the city in one afternoon only by walking. Of course, if you go to museums, the Opera or a theather, you should be there one more day. Not fascinating, but a fine city.
Insider Tip:Go to the parks. They are lovely and wide. The Ferris Wheel is a little more far, but the metro can take you there easily.