If in Yosemite valley, you could hike up to the top of Yosemite falls. Round trip is about 6-7 hours. Another one of our favorites is hiking from Glacier point down to the valley via the Panorama trail. It's about 8-9 miles but it's mostly downhill. It takes you right by both Nevada and Vernal falls and can be done in less then 5 hours. It also has spectacular views of the valley along the way. The only thing is you'll need to get dropped off at Glacier point. You could pay 20 for the shuttle to take you up there from the valley. As mentioned already, Tuolumne meadows is beautiful with the Lembert dome hike taking only 1-2 hours round trip. Another favorite is hiking from TM to the high sierra camp of Glen Aulen. If I remember that took us about 6 hours and covered about 8 miles. Have a great time!