Travel Advise for Europe

Will be travelling in Belgium and plan to travel from Brussels to Paris (via train) over the course of 7 days. Initially, I was thinking we'd head to Diekirch for 3 nights (good train from Brussels) and hit up Vianden Castle, do some hiking, and maybe a kayak. Then return to Luxembourg city for 2 days, stop in Nancy (or Metz) for 2 nights, then off to Paris to conclude the trip. We will be doing enough museums and art galleries (we actually start our trip in London and will have a couple days in Paris, too), so looking for hiking, neat outdoor experiences, and/or something really unique as per museums. Have seen some indications that Luxembourg is perhaps not that interesting.

Are there other suggestions for a good itinerary between these two cities with decent rail connections? Also hoping to spend at least 2 nights per location.