Walking across the river bridge is pretty nice. If you are a train buff, there are often ICE-3s cruising in and out of the train station, as well as the container vessels floating down the Rhein river..

The Dom (Cathedral) is a must see.

I'm a big Koelsch fan. As was mentioned earlier, it is served in a .2 liter glass, but it is a very fresh and good tasting beer. It is similar to a pilsner without the bitterness.

You are also about 30 minutes from Duesseldorf, which is a nice city in its own right.