Once you are in the city centre it's pretty to comfortable to just walk around. There's a main pedestrian area which is similar to an open-air mall, but more relaxed. Shops line the street and alleys, and it's mostly closed off to cars. Many shops, cafes and restaurants can be found in this area. Be sure to walk around the "Old City" (Altstadt) as well.
If you want to go further out of the city centre, you can use the developed bus system. Buses travel frequently, and on time! It is possible to buy tickets on the bus, just ask the driver. .