Welcome to opencarry.org! I'm also from the Boise area, maybe we should have an OC lunch with the others on here from our area? :P
I suggest you spend some time browsing the Idaho Statues here: http://www3.state.id.us/idstat/TOC/18033KTOC.html
most of them are easily understood by the "mere common man."
Other than that, I believe that OC at the State Fair is legal, though I'm not sure because it may fall under the private propert rights? I've heard of guys carrying OC or CC there before and were talked to by LE but not told to leave or disarm or anything... but that was a while back.
Open carry in Idaho is basically defined as visible to common observation or what a reasonable person would say was visible. So, don't throw you jacket over you shoulder holster... that would be CC.
An inside the waistband holster with just the grip of the firearm showing would be OC, because a normal reasonable person would know it's a gun, and be able to see it.
Basically use common sense.