Yellowstone closes trails annually to reduce encounters between bears and humans during certain times of the year. See here for bear closure info: https://www.nps.gov/yell/learn/management/bearclosures.htm
Additionally, roads in Yellowstone are closed annually during winter. Before you head out, see Park road closures here: https://www.nps.gov/yell/planyourvisit/parkroads.htm
Yellowstone National Park charges a fee to enter. Fees are $35 per vehicle or $30 per motorcycle. If you are entering on foot, horse, or bike the fee is $20 per person. You can also purchase a park specific annual pass for $70.
A nice hike continuing along Boundary Creek to the beautiful Dunanda Falls. This trail will take you to a view point above the falls but there is also a trail to the bottom of the falls where natural hot springs enter Boundary creek and make for a very nice warm bath.
This hike is deeper into the Yellowstone back country so care should be given in the advent a bear meeting arises. Deer and elk are plentiful here. You will see Silver Scarf falls from the trail but you have to do a little bushwhacking to get a good view. Also remember this is back country. There are no handrails or slip proof sidewalks at the top of the falls. Be sure of your footing at all times.