As others have said, the Mononogahela Incline is a must see. If you're in the 'burgh, you gotta check the box.
Although this is considered a landmark and an attraction, real people use this thing to go from homes on top of the hill to the transit system and the city below just as was done over a hundred years ago: "Look kids!! Rrreeal people use this! Don't get too close, though, they might bite."
It is conveniently accessible from Station Square (which is not a must-see). It enables easy access (for a fare) to the top of Mt Washington which provides one of the most photographed and beautiful panoramic city views I know.