You mean Jump Creek Falls? I don't think of that as a good camping spot. But nice hiking if you know how to identify poisonous plants:
Note that there are bans on burning, and the weather here is chance of t-storms. And it's hot here.
I'd head up high to Mores Mountain campground. I think it's $5 to camp there? Should be ~15° cooler than in Boise. Of course, you might also get some lightning...
There are some spots in the foothills, but I'll bet with the current fire hazard the authorities are out and you'll get booted, if not ticketed.
So there is always KOA....