The National Arts Centre is Canada's main performing arts venue, a brutalist concrete complex at the edge of the Rideau Canal that houses opera, ballet, theatre and orchestral performances. The building was completed in 1969 and has recently been renovated — the new glass addition on Elgin Street has opened up the formerly opaque facade and connected the building to the street in a way the original design didn't. The resident National Arts Centre Orchestra is one of Canada's finest.
The building itself is worth a look even if you don't attend a performance: the terraces above the canal are a good spot to sit in summer, and the internal layout of interconnecting hexagonal lobbies is impressive. The restaurant overlooking the canal (now independently operated) has one of the better views in the city. Check the programme before you visit — the NAC brings in major international companies for its opera and dance seasons, and catching a performance here is one of the city's genuine highlights.