Bear Valley Hot Springs
I hiked with my dog Ginger to what has to be the nicest hot spring in Idaho, and Idaho has a lot of hot springs. It was a 3 mile walk from Fir Creek campground, but I would not recommend the hike for kids or anyone with balance problems. There was a part where the trail was precipitous, and a fall might have been fatal.
There is a different trail that is safer, but requires a fording of the stream. It is unfordable this time of year. The picture below is of the best pool, which is right next to the river. The water in this pool was a uniform 104 degrees, and was clean with a gravel bottom and no algae to speak of.
Below is where the hot water comes down from the right, into a stream that is hot water only. Next to the hot water source is the hottest, and the pool before it enters the creek is 104 degrees. Many hot springs have a hot water source which must be mixed with cold water, and its not unusual to be cold on one side and hot on the other. These springs have only hot water, with the coolest being about 104. There are a number of pools, but the one closest to the river is by far the best, and could hold probably 10 people, in water up to mid thigh in depth. There are maybe places for about 6 tents.
The steep part of the trail is about like this (about 6" wide), but with some down trees on certain parts, and a nasty drop off if you trip or slip.