Boiling River is an off-map must. This five-star hot tub, one of only two safe (and legal) soaks in Yellowstone, offers a fun break from family touring. Amble the paved ½-mile trail. Watch birds, bison and elk. Soak in a natural spa, a mix of hot (hot spring stream) and cold (Gardner River). But before you pack your suit, water shoes and towel take note of the following things. The rocky riverbed is often slick. The water isn't potable. Boiling River is the worst-kept secret in Yellowstone; there are crowds on hot summer afternoons.