I think that spending at least one day in Brussels would be very rewarding also. The historical center is impressive and the city has some of the best museums in Europe.
Around Brugge there are very fine bicycle trips along canals that I like a lot. They provide nice photograph opportunities on the low lands.
If you have the chance to travel by car, there are some worthwile castles to visit on the way to Luxembourg, my personnal favourite being a small castle called "Chateau de Modave" famous for having been the test ground for the Versailles garden hydraulic systems.
http://www.modave-castle.be/Anglais/Intro.htmlI would also try to arrange a visit to a beer brewery. You have several options available in different parts of the country.
Have fun.
Cheers,
Bernard