This is one of my favorite places to visit in this area. The city was founded by the Romans in 15 BC and is the oldest town in Germany. The Porta Nigra, "Black Gate", is the dominate landmark of the city, dating back to the second century as the northern entrance to the city. For a small fee you can go into the Porta Nigra and see a great view of the city.
There are many markets in the city, especially during the holidays. There are also Roman baths, the oldest Christian church in Germany, and the Royal Imperial Palace to visit.
Many places to eat and many hotels or cute B and Bs to stay at.