Although Luxembourg is considered to be a somewhat dull city in Belgium, we visited the city center (Oberstadt) a couple of years ago and found it quite nice.
So a few weeks ago we drove to Luxembourg again, this time to do the Wenzel walk. This approx. 5km circular walk mainly leads you through the Grund quarter, along the city's fortifications / old city walls and also includes a number of good viewpoints.
With some time left, we also visited the Grand-Ducal's palace which is possible by guided tour in the summer months (but no pictures allowed inside).
Luxembourg is not a big city but certainly worth a one or two day visit.