Natural History Museum here in Washington, DC. The Hall of Mammals was impressive, as were the Mineral and Gemstone Galleries and the Iceland Revealed Exhibit.
Nature's Best Photography Exhibit showing on the 2nd floor. I don't believe I've ever seen a collection of such stunning, HD-quality, wall-sized photos in my life. I practically cried. If you have a love for nature photography as I do, you'll want to see this.