Definitely a tripod. The valley gets dark fast in the evenings but maintains a great glow, so you can do some of your best work with slow shutter speeds. Night photos are a great source of material too - bus stops, camp fires and public spaces. There are also a lot of places in deep shadow during the day. I would recommend a gradient filter if you feel like messing with such a thing - the exposure difference between sky and valley is drastic. The waterfalls are usually pretty low in August, but take a camera cover just-in-case. The Ansel Adams Gallery sells these, as well as filters and film. Their prices on film are actually really good. August is VERY busy in Yosemite, like NYC levels of people. Study a trail book before you go and spend time above the valley floor. Finally, mosquito repellent and gear for very hot weather. Have fun!