The Cathedral of Learning is a beautiful and unique treasure. If you're visiting the city and are looking for something different to do, definitely consider touring the Nationality Rooms. $4 for adults and $2 for children, and you can explore the amazingly curated classrooms that are decorated using the cultural aesthetics of different countries from around the world. Some of the rooms are more impressive than others, but in terms of a distinctively unique experience I can't speak more highly of the tour.
The main hall of the Cathedral well...it looks like the Great Hall at Hogwarts. Which is pretty cool if you're a Harry Potter nerd, and even cooler if you're a Harry Potter nerd that attends The University of Pittsburgh. I mean, or if you just appreciate beautiful architecture. I guess.
During the winter holidays each of the rooms are decked out in the traditional holiday decorations for that culture. I would highly recommend touring during the Holiday Open House (typically at the beginning of December) - that's the only time during the year that they open up the Early American room (which has a secret upstairs room!) and the Syrian Lebanon room for the public. There are vendors, lots of food, dancing, music, and tons of activities. It can get a little congested but it's definitely worth checking out - and it's free!
Anyhow, the Cathedral of Learning forever holds a special place in my heart. A+ job Pittsburgh.