We only stayed one night and left early the next morning. There's an English tour you can take of the Koln Cathedral, if you're interested in a bit of history. I had the Cathedral on my list as a must-see, so since we arrived in time, we did the English tour (3 PM), and my husband enjoyed the information, as well. He LOVED the Brauhaus for kolsch, I think it was Fruh that we visited. We did go on a Friday and there was music by a local folk singer -- we learned that all the songs are about how special Cologne is. Later on the Rhine, we met an older couple whom the woman was from Cologne, and you could hear her love for her hometown.
A walk across the bridge to watch the sun set behind the Cathedral is an easy activity. Bars across the river for a beverage, or BYOB.
I would have loved to have had time to visit the churches aforementioned. We visited the cemetery instead (the only cemetery we saw in Europe).
My mother in law loved doing a bike tour of Cologne.
We rented a car and drive south on the Rhine, first driving to Burg Eltz castle. The small bit of the Moselle River area we saw was lovely -- next time we go, we will visit that area, though we loved the area we stayed on the Rhine, too. We never made it to Marksburg castle, but we did do the ruins one (Rheinfels?) in St Goar.