Romanian National Opera shows Bucharest through performance rather than monuments. Depending on the calendar, it can mean a formal evening out, a festival crowd, or a glimpse of the local arts scene that guidebook routes often miss.
Check the programme before you go; the building or event is most rewarding when there is something on. Even if you only stop by in daylight, it helps anchor the city as a lived-in cultural place rather than a set of static sights.