Phipps Conservatory will lower your stress level the moment you step inside. The newly renovated botanical gardens are simply beautiful. Individual rooms are themed using indigenous plants, water features, and climate control to simulate deserts, rain forests, and swamps. The plants are all well-marked, and even if you aren't a plant and flower lover, you'll like it here and learn a lot. Admission is quite reasonable, and students get a discount/free admission by showing their ID.
I recommend visiting during the winter, when their candlelight tours take place. Hundreds of candle-filled lanterns fill the walkways, and local musicians perform during evening hours.