Lufthansa Airlines and other Star Alliance member airlines (Air Canada, United) have roundtrip airfares from Denver CO (DEN) to Luxemborg City (LUX) for $381 or Brussels $373, including taxes and fees. This makes 1 stop in Munich Germany.
Sample Travel Availability:
For Luxemborg on Lufthansa Airlines
Departs DEN: Feb 1, 3, 4, 7, 8, 10, 21, 22, 24, 25, 28; Mar 1, 3, 7, 8, 14, 15,
For Brussels
Departs DEN: wide availability Feb-Mar. Use google flight search
My sample travel dates: Feb 1-8, 2017