Roystone hot springs is the closest hot springs to where I live (it's between Emmett & Horseshoe Bend) so I had to check it out! I went with a church group so it was all paid for but normally a tent site is $10 and I'm not sure if it's an extra charge to use the pool. The hot springs pool was open air but has a roof (nice for the shade), made of concrete, and perfect temperature (it was a hot day but they keep the water probably a little cooler than body temperature if the weather is hot). As far as the camping, the amenities are nice but there is not much privacy (sites are all in one big open grassy area), and little to no shade. For that reason, I wouldn't recommend this spot in the heat of the summer. This is definitely a more commercial option as far as hot springs camping goes, which I'm not a huge fan of, but overall it's a well-kept place and I might do some camping there again in the fall. It is very conveniently located.
I have camped a couple times at Hot Springs campground. Pros are that it is easy to get to (campground is literally right off the highway), only about an hour and 20 minutes from Boise, and the amenities are nice (fire pits, tables, bathrooms, etc). The best part about this campground is it is right across the highway from an awesome little Hot Springs pool overlooking the river. Easy access any time of year. The main reason I gave it 3 stars is because I like to feel a little more secluded / not right beside the highway when I camp. On that note… Cons are that since it is so easy to access, it can often be very crowded, especially on weekends obviously. You do have to pay for parking / camping, and it is not very secluded - you can hear cars passing on the highway. There is some shade but not on every site. In my opinion, this campground would be best to camp at in early spring or fall.
Arrowrock Reservoir is a decent place to camp. The drive there is long and slow-going along a washboard dirt road, but the best part is that this spot is usually uncrowded, which is nice when you are looking for some quiet solitude. It's a steep hike down to the water in the reservoir, but not too bad. The camp sites are nice and have some pine trees, but other than that, the landscape is very desert-y.
Redfish Lake is one of the most beautiful blue lakes in Idaho. We went there on a weekday in May, and there was hardly anybody there. I know weekends can get pretty crowded, but this is such an amazing place to camp, hike, and relax. The water is definitely cold, but it is absolutely beautiful. Highly recommend.
The salmon river is absolutely gorgeous. It has some nice swimming holes or eddies along the river, and the water is perfect for swimming in late summer, though a little chilly in the early summer months. The white sand you can find along the river is soft and beautiful.
Kirkham Hot Springs / Campground is one of our favorite places to camp. Although I wish there were more trees (it gets pretty hot when the sun is out), this is an excellent spot for camping in the fall. It's a somewhat small campground, and the best part about it is it has easy access to hot springs. There are natural pools right along the campground itself, or if you want to hike down along the river (I use the term "hike" very lightly here - it's a quick walk, but along rocks / boulders), the natural pools of hot water come trickling off the hillside and pool up right next to the Payette River. It is beautiful.