I think anyone who lives in Porto, can guess we're this is. My favorite place in the city. It's so beautiful. Many people don't know this, but you can just come right in and wonder around. You don't need to pay, or have a specific reason to go there.
Of course if you're not from here I would recommend you visit Casa da Musica, but with a tour guide. It's not expensive at all (about 3 €). Just so you don't get lost, and understand what the building is all about.
When we don't really know we're to shoot, we just go there. It has a great view of the city and really cool rooms. Also I think they still have lots of Machintosh, for public use but you can only produce music, which makes sense since it's "the house of music". I tried it out once and it was fun. Of course, they also have great concerts of all types of music.
So that's my suggestion, everybody have a good week!