You have to see the color of Diablo Lake to believe it—it's a breathtaking, milky-turquoise that seems to glow, fed by the surrounding glaciers. The best view is just steps from the parking lot at the Diablo Lake Overlook, where the jagged peaks of the North Cascades frame the water perfectly. For a closer look, you can take a moderate 3.5-mile roundtrip hike on the Thunder Knob Trail for an even more spectacular vantage point, or even get out on the water with a kayak.