Yes, it’s possible to visit Luxembourg for one day. The city is just too small that you don’t even have to stay there for a night. I’ve seen and did all the things I wanted to do in less than 6 hours. I could have taken a train to Brussels that evening but I already booked a hotel, so I had to stay.
My train from Paris arrived in Luxembourg (2 and a half hours) around noon. As soon as I exited the train, I directly went to my hotel, right in front of the station. My room was already cleaned up so they allowed me to check-in earlier than scheduled. I rested for an hour and off I started my sightseeing.
Things to do in Luxembourg for One Day
Travel Tip for self-guided Tour: Find the following places on your Google Map app before you embark on your own walking tour. You don’t have to follow this route. Most of these attractions are just meters away from each other, so there’s no need to take public transport.