I was underwhelmed by the 90-minute ? boat tour I took. This is not a picturesque, historic part of the Rhine, it's modern and industrial. Walking over the river on one bridge, and back on the one that leads to the modern art museums is a nice outing. I'd skip the Chocolate and Cologne (fragrance, that is) museums, but be sure to go to a Kolsch bar, any of them. Even if you don't drink beer, you have to see the small wooden barrels being tapped and served.
You don't give your personal interests. Cologne is FAMOUS for its museums, and there is something for everyone. For example, the Applied Art (MAK) museum is perfect for those who don't much care for acres of paintings on walls. The Ethnographic museum is edgy and modern, and is neighbored by a superb Medieval Art museum in a deconsecrated church. (metro train to this pair.) Do you know that the Cathedral (Dom) is the most-visited monument in Germany? The extra-cost treasury is nice, and very cool on a hot day. Look way up in the Cathedral for Gerhard Richter's abstract stained glass windows.
I liked the very famous Roman-Germanic museum. There is also a very unusual Roman monument (among many in Cologne) partly underground, and with an optional tunnel to explore, the Praetorium. There are some special archaeological digs for very old Jewish Cologne, if that interests you. I thought the Documentation Center (i.e. Nazi era) museum was not so special, they're a dime a dozen in Germany, so it doesn't matter which one you go to if you have limited time. If you have a day for a concentration camp, that's important, elsewhere.