Best way of going to Sibiu is by either your own car or by train. For travellers from abroad they should choose an intercity because the quality of other Romanian trains lacks a lot.
Best way is with your own car. But you can also make use of the buses in the city. Some years ago these buses had a bad reputation for not being in time, but maybe this emproved. Anyway you only need the bus to go to the Museum of Romanian Village and to the Zoo. To see the old city you only need to walk.