Denmark's behemothic Opera House sits on Copenhagen Harbour, directly across from the Amalienborg Slot. Completed in 2005 and embellished with limestone, marble and gold leaf, the Henning Larsen–designed building spans 4100 sq metres and includes 1000 rooms. It's also one of the most expensive opera houses ever built, costing a cool one billon euros. The lobby houses glass sculptures by eminent Icelandic artist Olafur Eliasson and guided tours are available in English and Danish; tickets can be purchased online.
See the website for tour times and days.