Daugavpils Musical and Drama Theater is the city's principal performing-arts venue, housed in the central cultural complex on Vienibas Square. It adds a live-culture option to a city itinerary otherwise dominated by the fortress, churches, and the Mark Rothko Art Centre.
The theatre is most useful if you can match a performance to your dates. Even without a show, its central location makes it part of the civic core and a good marker for exploring the surrounding streets.