Yosemite? Apparently it can be done, but it is VERY tough.
Stay at the Yosemite Bug hostel (closest and best cheap accomodation - book in advance tho') then check the weather for your planned visit and try. At least go up to the waterfall, a good few hours hike up and down to the top and back. That's a great hike with brilliant views.
http://www.yosemitebug.com/
edit - can't tell you how useful the California rough guide was btw.
