I liked Trier and was surprised by the amount of high quality monuments in the city. The Porta Negra is very impressive, the Dom is stunning for its importance and its holy relic, a tunic supposedly worn by Christ. Then there is the Basilica of Constantine which is impressive for its age, the massive interior and architectural importance as being a forbearer to the Romanesque movement. The Amphitheatre is a little way out side the city walls but is quite large and provides a great place to sit and have a sandwich or run around like a school child (I opted for both with the emphasis a bit too much on the later!)

Not part of the UNESCO site is the birth place of Karl Marx! There is not much there, but it is nice to see none the less.

Trier has enough to occupy you for a day trip and makes a great one from Luxembourg as it is only about 30 minutes by train.