Both are excellent parks if you haven't view the NPS web site for short hikes in yellowstone, especially the canyon are, the falls at yellowstone are really great if its been wet winter
Yellowstone National Park - Trip Planners to Specific Park Areas (U.S. National Park Service)
The park is BIG, better to hit some spots then try for all see bison note below
At grand teton, take the ride and hike at Jenny lake
for both parks have the camera ready lots of wildlife, although by the 10th time that the bison block the road in yellowstone it does get a little old, some days they did not block the road at all.