Luxembourg is a place people most skip as they travel. It is to small and unimportant to simply travel to Luxembourg, and therefore they go somewhere else instead. The ideal way to visit Luxembourg is though to take a day trip while in for example Brussels. Luxembourg (the city) is about 3 hours distance with train from Brussels, and while visiting the city you can see a nice city center, nice architecture and after 6-7 hours you can say that you have seen, been to and visited Luxembourg and go back to Brussels with a good conscience.
If you want to join in on an organized tour to Luxembourg you can do so with Viator.