I forgive you for doing all caps. My wife does it too even though I try to explain all caps is the equivalent of yelling on the internet.
Berlin from Koln is like taking a daytrip from San Francisco to Los Angeles. It's a long trip and there is too much to see for a day trip.
Possible daytrips from Koln by train.
Down the Rhein to Bacharach and St. Goar, use your train pass to take the river cruise as part of your daytrip. Marksburg castle and Rheinfels ruins (this will be a long day if you do everything).
Down to Cochem and Trier with a stop for Burg Eltz (Not quite as big of day trip).
Bonn for a day (easy trip).
Aachen, Maastricht and/or Brussels (can be easy or long depending on how many stops and what you decide to see).
There is at least one nice castle in the area with surrounding park and gardens (I can't remember the name off hand).
There are several Bads (spa towns) within daytrip range. Most have co-ed no clothing bathing. A few allow bathing suits. Don't go if the nudity thing will be overly uncomfortable.
It's hard to do too much recommending without knowing what you are interested in.
Remember that the faster trains don't stop at the smaller towns. A good technique is to take the fast train to the furthest point in the day then work back on local trains. When you have had enough you can head back to your hotel without any more stops.
A stop at the tourist information office in Koln is a good start. Tell them the kinds of things you like and let them make some recommendations.