This is such a great place to visit!
Almost every time I visit I tend to stop at Cafe Phipps for something to drink or a meal. Being that it's a "Garden", during harvest season some of the veggies actually come from their gardens! Everything served is healthy and locally grown and organic. But I absolutely love just stopping to get a Phipps Splash. They are so yummy and refreshing!
So they change their exhibits up several times a year (would like to say once a season, but I may be wrong). But my all time favorite show is the winter show. It is absolutely stunning. Everything from the Orchid Room to the outside light show! During the winter exhibits they tend to be open until 11pm (just something to keep in mind). Also this year you could reserve your time (I can't remember if this is new or not) so you didn't have to wait in those outrageously long lines.
I would definitely put this on a to-do list if you need to do something in Pittsburgh.