Koln to Interlaken

The quickest route is rather scenic in certain areas, so you may as well take that. South of Köln, link up with the A61, and take that to Karlsruhe, Take the A5 from there to Basel. From there, follow Swiss routes 2,1 and 6 to Interlaken. A61 closely parallels the Rhine gorge, so you can easily take a quick detour to descend into one of the towns along the Rhine. Also easily accessible is the beautiful lake front abbey of Maria Laach. You probably could drive it one long day, but it would probably be more enjoyable to break in two. For towns to spend the night...Baden-Baden is one of those places I always wonder why Rick Steves' makes such a big deal out of (if you have to see a spa town, I would normally recommend Bad Dürkheim or Wiesbaden first)... but it would make a very convenient over-night stop for you. It's not worth going out of your way to see it, but not a bad place if it lies along your path of travel. Other good options would be Karlsruhe (huge baroque schloss), Freiburg, or even Strasbourg in France.

No sense in replying or elaborating - Tom has said it all, very well may I add.