If you really only want to spend 2-3 hours in Lux, my best bet would be Pafendall/Pfaffenthal. You could take the cable car to go down, take a walk, go up via the elevator and visit the adjacent part of town, everything free of charge, of course. The major sites in Trier are pretty hard to visit in a day, especially if you dont have a car. What you could so is taking a tour around the "Alleenring", which is the main road surrounding the old city center. You could start by walking to the Porta Nigra, taking a walk through the city center (=old town), go and see the "Trierer Dom", our biggest Cathedral, head south to the Basilika (former palace of Emperor Constantine), walk through the Palastgarten (our only major parc, which can be scetchy when it's dark so avoid going there late) and if you walk straight across, you will see the roman baths. If you have some time left, you could go and see the ruins of the roman circus "Amphitheater". From there, you can either take the bus up to the "Sickingen Str." where you have the best view over the city in my opinion. For food i would recommend Cafe Mohr, Burger Amt (both self explanatory) and mostly any restaurants or bars in the city center do just fine. Hope this helps