1) The Fairy Falls to Mystic Falls Loop hike to two secluded awesome waterfalls. In the same area is the Grand Prismatic Spring Overlook. 2) The hike out to Lone Star Geyser is flat and if you have bicycles I believe you can ride them out and back. 3) Near Mammoth Springs, you have two options 3a) the trail to the top of Brunson Peak or 3b) the trail to Osprey Falls. Finally 4) by Canyon is the Mt Washburn trail. 5) Not really a hike, but you can spend an entire day in the Upper Geyser Basin (Old Faithful area) if you take your time and are patient you will see several of the big geysers erupt. Check at the visitor center for estimated times and plan your geyser viewing from there. Keep in mind that in Yellowstone you are in bear country and some or all you should be carrying bear spray (and know how to use it - so practice before).