Take a ride up the Duquesne Incline, a funicilar built in 1877, for great views of downtown Pittsburgh and the intersection of the three rivers.
The round trip ticket costs $5 for an adult and requires exact change. (A five dollar bill will do the job.)
There's not much to do at the top, at least not right nearby so most people just ride up, look at the city and take pictures from the observation deck, and ride back down. The station house at the top is basically a tiny museum with the history of the funicilar and of Pittsburgh. It also has a little gift shop.