Downtown Pittsburgh has quite a few nice little squares. You can sit and eat your lunch, socialize with your friends or co-workers, read a book, or just people -watch. I think that of all the squares Downtown, Mellon is my favorite. It doesn't look like much from the Smithfield Street side. Indeed, because the Smitfield Street side is much lower than the William Penn Place side, there are a few ugly strip-mall type stores in there including the Port Authority Tranist (PAT) ticket office.
There is also a parking garage underneath, accessible from Sixth Avenue.
But the plaza above it is really nice, and it's the perfect place to while away a little time if you need to get out of the office.