The reason I chose mirrors is because one of the main themes of Paper Towns is that people are not always what they appear and that they can look better than they are. This quote, "The longer I do my job, the more I realize that humans lack good mirrors. It's so hard for anyone to show us how we look, and so hard for us to show anyone how we feel." Represents that nobody can quite know a person completely and nobody can tell someone who they are and how they are feeling.
Q's window is important because its where Q and Margo are reunited and where Margo asked Q to go on an adventure the night before she left. "I swiveled around when I heard the window open, and Margo's blue eyes were staring back at my." That was a quote from the first time they interacted since they found the dead mans body together when they were kids. Little did Q know the adventure they were about to go on would change his life forever.
One of the funniest parts of the book in my opinion is when we find out that Q's friend's parents have the world's largest collection of black santas. “IT'S NOT MY FAULT THAT MY PARENTS OWN THE WORLD'S LARGEST COLLECTION OF BLACK SANTAS.” I think this is symbolism because it shows yet again that not everyone is what they seem. Everyone just assumed Santa was white. It's also reminds readers to keep an open mind.