Vienna has the most incredible public transport system; both the tram network and the metro. This is the No.1 tram. If use both No.1 and No.2, you can do a complete loop around the Ringstrasse (Ring Road), what was built to replace the city walls. There are many historic sights on the way, including the State Park, the Opera House and the Franz Schubert.
If you are feeling fit, it is a nice 5k loop along the same route (of course on the pavement next to the trams).