Yosemite Valley, Half Dome, and the high country from Glacier Point.
If you only have one day in Yosemite in the spring, fall, and winter where do you go? The answer is easy, tour the loop of the Valley floor and stop to see Bridal Veil Falls and Yosemite Falls. From June to early August, depending on the snow pack and the run off, you can jump in the Merced River for a swim. In winter you can ski at Badger Pass.
In the late Summer those iconic Yosemite waterfalls tend to dry up and the lower Merced River can slow down to the point it gets green and smells more like a lake than a mountain stream. Your best bet, in dry years like this year, would be a drive around the Yosemite Valley floor then head up to Glacier Point to watch the sunset and stars over the Yosemite Valley. Or you can drive up Tioga Pass and check out Olmsted Point, Tenaya Lake, and Tuolumne Meadows.