If you only have one day in Luxembourg, you should spend it in Luxembourg City. You will most likely arrive in Luxembourg City, and rather than spend more travel time getting to and from another area, you should just focus on the capital city. There’s plenty there to keep you busy.
First thing in the morning (hopefully your hotel will offer breakfast), head to the Bock Casemates. These fortifications were part of the castle that Count Siegfried built in 963, essentially founding the nation of Luxembourg. The city’s fortifications and underground tunnels extend many miles over the Alzette Valley, so explore as much as you want, making sure to stop at the bridge over the Alzette River for a great view of the area.
Next, head downtown and tour the Palais Grand Ducal, where Grand Duke Henri lives. Depending on your time, you’ll want to get some lunch either before or after touring the palace. Grab something near the palace or head a couple blocks west to Place d’Armes, where there is food and tourist information.
After lunch, you have several options. If you like museums, you should consider either the National Art History Museum or the Museum of the History of Luxembourg City. On the other hand, if you prefer history, consider either the Cathedrale Notre-Dame or a further touring of the city’s medieval fortifications, especially the Petrusse Casemates, which date to the seventeenth century. Each of these attractions is within close walking distance of the others, so you could very well have time for more than one, but you will not have time for all four.
Spend the afternoon on these sites and then return to your hotel to rest for a little while. Then, if you’re a gourmet food aficionado, consider one of Luxembourg’s fabulous restaurants. A word of caution: Luxembourg restaurants can be very pricey, so go with a local recommendation rather than risking your money at a random restaurant. Even if you don’t go out for dinner, however, go explore the Place d’Armes at night; it’s a treat to see the activity of a small European capital.