Nice's opera house is a splendid example of Second Empire architecture, inaugurated in 1885 on the site of an earlier theatre that burned down during a performance of Lucrezia Borgia. The auditorium seats 1,050 and has excellent acoustics; the season runs from October to May with a mix of opera, ballet, and orchestral concerts.
The exterior facade on the Quai des États-Unis is handsomely floodlit at night. Even if you are not attending a performance, the building is worth looking at from the outside, and guided tours of the interior are available on certain dates. Tickets are reasonably priced by European opera standards — check the program as performances by the resident orchestra and visiting companies are often excellent value.