The upper falls trail is nice, but it can get slippery near the top; crampons or other traction devices are a good thing to have handy. Also, if there's fresh snow at the top you may not be able to be able to see the trail. Unless you really know where you're going, don't be reluctant to turn back.
The winter route up the John Muir trail to Clark point is nice, and really a sight if it's snowing in the hills around Nevada Fall. You can continue down some switchbacks to the mist trail from there and go on up to Nevada Fall if it's not icy.
The Tenaya Canyon Mirror Lake loop to Snow Creek is always a nice walk. I haven't been on the Old Big Oak Flat road or Old Wawona Road trails in winter, but plan to showshoe up the old Wawona this winter if possible.
Also, not in the valley, but the road to Mariposa grove is a really nice snowshoe walk when there's lots of snow. There's usually a ski trail, but snowshoes allow you more freedom.
With fresh snow, most any valley trail is great; the valley loop that goes from Bridalveil up toward the four mile trail and on is nice in the snow.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo