The opera house is one of the most famous in the world. Take a tour inside, it’s only 4 Euros for the student pricing and they don’t even check IDs!!! Side note: buy a ticket for the standing seating for one of the concerts, they usually have one every day. Ticket prices are usually 2-3 euros , but be prepared to get there early and stand for the entirety of the performance. You may not have a great view, but you can’t beat that price!!