Your ship will dock right at the city's edge, just minutes from the Stone Bridge, which was completed in 1146 and is arguably one of Regensburg's most famous attractions. Another important site is St. Peter, a Gothic cathedral built between 1274 and 1520. If you're a BMW fan, you might also know Regensburg as home to the production facility for several of its models. Ask if your ship offers excursions to BMW's factory.
Most of your time, though, could easily be spent wandering the narrow lanes that are lined by merchants' and craftsmen's houses. There, you'll find spices, pralines, handicrafts and jewelry. You should also visit one of the many beer gardens in the city, many of which feature hidden patios in the back. Enjoy a cold brew, perhaps accompanied by a Regensburg sausage.