I would stick to Amsterdam and Cologne. Easy train ride. Take a Rhine cruise as suggested. Some hiking opportunities outside of Cologne but I also enjoyed just hanging around Cologne walking around and drinking with the locals on Saturday afternoon in the old town. If you get bored with Amsterdam (not sure how that could happen), then some day trips on bikes from Amsterdam. Both have great food and both have great beers. I think you could easily fill 9 days with those two cities.