"Would you suggest staying in Cologne and doing a day trip down to Bastogne and then several other day trips into the Netherlands"
I struggle to understand why you'd base in Cologne if your goal is to visit "the Netherlands" or Bastogne (4+ hours of driving round trip) on day trips.
A very nice city in the Netherlands named Maastricht could possibly make for an enjoyable stay as well as a viable base town for visiting Bastogne and some of the nearby Ardennes towns, as well as NL towns of interest nearby (like Valkenburg) and certain Belgian towns (like Namur and Dinant) and Monschau (one of Germany's finer small towns.) We stayed in Monschau once and visited the Henri Chapelle cemetery on a day outing. If you feel you need to stay in Germany, I suppose you could base in Monschau, or Aachen perhaps (also a nice destination) as both are closer than Cologne to your other named daytrip destinations - that is, if your driving distances work out from these towns (check viamichelin.) Aachen as a base would permit you an easy day trip to Maastricht and into Cologne as well (if Cologne was to be not just a base town but also a sightseeing destination.)
Remagen, Germany is a good place to visit for WW II history. It's south of Cologne on the Rhine. Perhaps it's a day trip from Aachen or Monschau, or perhaps you'd visit it on your approach to the area you're targeting.
