Mammoth campground is the only campground open all year round. It's not as big as Bridge Bay or Grant Village, in fact it is one of the smallest campgrounds in Yellowstone National Park. It is right by the Mammoth Hot Springs Hotel, the Albright Visitor Center and the North Entrance. There are quite a few short hikes nearby. If you want an easy one mile hike checkout the Boiling River. The parking area is just north of the Mammoth Hot Springs. It's Yellowstone, no campground here can be less than four stars!