Tim,

As I mentioned above, the nature centers in Yellowstone have a pamphlet with called Easy Day Hikes or similar, which indicate distance and terrain. The centers should also show areas that may be closed due to bear problems, fires, etc. Two walks that come to mind are Lone Star (near Old Faithful area) and Cascade Lake (IIRC) near Canyon.

Check out the day hike information on this page: http://www.nps.gov/yell/planyourvisit/hiking.htm