It depends on when you're going. The Tioga and Glacier Point roads will likely be closing in a few weeks, which will wall off a big chunk of Yosemite from you. Yosemite Valley is open year round, though, and is full of great stuff -- the tunnel view, El Capitan and Half Dome, the Mist Trail, Bridalveil and Yosemite falls (though waterfalls won't be at their peak at this time of year). It's less crowded after Labor Day, too.
The Mariposa Grove of giant sequoias, in southern Yosemite, should also be open. (It's open year round, although during the peak of winter you'll need snowshoes.) If you go before Tioga & Glacier roads close, you won't regret visiting Glacier Point or the highlights of the Tioga Road area, like Tuolumne Meadows and Tenaya Lake.
more info about visiting Yosemite in autumn: http://www.yosemitehikes.com/yosemite-info/yosemite-in-autumn.htm
overview of Yosemite's highlights: http://www.yosemitehikes.com/yosemite-info/greatest-hits.htm
some suggested Yosemite itineraries: http://www.yosemitehikes.com/yosemite-tour/yosemite-tour.htm