Let Yellowstone Trails: A Hiking Guide by Mark C. Marshall (park ranger) get you started. This was published by the Yellowstone Association and covers both short and long hikes. I'd recommend the Bliss pass in the Northwest via Slough and Pebble creek as one of the tip top all day hikes.
Yellowstone: Image taken by dmdzine, discovered on www.flickr.com.
Here we have had snow and temps below freezing for a few days, but my favorite time in Montana is fall, hopefully it will come back for a few days for another hike or two.
Missing summer already,
Catherine @ Reading In Montana