How about Eltz castle? Is that a half day trip?
That would be a pretty long day. According the the Bahn website, the trip from Cologne to Moselkern is about 2 hours each way.
According to another post on this site, allow about 1½ hours each way for the walk between Moselkern and Burg Eltz and an hour for the tour. Total time, Cologne to Burg Eltz, tour, and Burg Eltz back to Cologne would be 8 hours.
According to ViaMichelin, the trip from Cologne to Burg Eltz by car is about 1½ hour each way, 3 hours round trip. So the total trip could be 4 hours. But, unless you already have a car, you'll have to add the time to get to a rental shop, rent the car, then take the car back, and I don't think Burg Eltz alone would be worth the cost of a car rental for a day.
Also according to the Bahn, the trip from Cologne to Braubach (Marksburg) is about 1h15m, something like 2½ hrs RT. The walk from the train station to the Marksburg is much shorter than the walk to Burg Eltz, and, if you walk down to the K-D dock, you might catch the shuttle to the castle.
The Marksburg is the only castle on the Rhein that was never destroyed and the German Castle Verein has preserved it as it was hundreds of years ago.