Stockholm’s Opera (Operan) dates to the late 18th century and is the national venue for opera and ballet. The current building was constructed in the late 19th century and was rejuvinated in the late 20th century. It’s located on the Norrström river, just across from the Royal Palace (it serves as the Royal Swedish Opera, and the royal family has its own box).
I visited the building at dusk. I’m not a big fan of Opera but found the exterior to be quite appealing. Tours are available of the interior if that interests you and you might be able to snag a ticket to a performance if that coincides with your visit.
Facing the Opera is a statue of King Gustav II Adolf which was erected in 1796.