You could stop at the Kröller Müller museum in Hoge Veluwe national park. It has statues in a garden and multiple Van Gogh paintings in the museum. The national park is foresty (not spectacular by international standards) and you may spot a deer.
I also like the Millinger Theetuin tea garden and hortus. But that would be a stop quite close to Kröller Müller (you won't make much progress that way ;-)).
I hope you have accommodation sorted for Amsterdam, mind the IBC conference!
Ask the car rental company about the environmental sticker you will need for Cologne.Edited: 17 February 2015, 23:52
We plan to leave the car at a Park and ride and not drive into the center of Cologne. I have been tryong to figure out if it is ok to leave the car in the Park and ride for 48 hours. Do you think we need the sticker if we do not bring the car into downtown? Thanks for any insight.
You didn´t answer to the question, if you just want to do a short stop anywhere or do you even plan an overnight stay in another destination.
And what about your main interests? History, museums, monuments of the Roman imperial period, industrial areas and museums, small scenic towns eg. with half timber houses (Kettwig/Essen)?
And there is Dusseldorf, not to forget...but it´s too large just for a short stop and you would need a sticker as well.Edited: 18 February 2015, 00:26
Just looking for a couple hour stop along the way to break up the drive.
You won't need the sticker if you don't drive into the city itself.
I accidentally drove my old car into central Münster which also has environmental zones (unplanned visit, I didn't know). No one stopped me but I guess I could have gotten a fine.
For a few hours stop I stick with my suggestion for Kröller Müller.