If you have a friend visiting from out of state and you let them drive your car to a museum or something, as a one-off thing, I think it's a little impractical to let your insurance provider know that your friend is driving the vehicle, but practicality aside, would informing them be the proper thing to do?

Like what happens if your friend is driving your vehicle and they get in an accident? If there was a serious threat of that then letting the insurance company know might be prudent, but then again, maybe just not lending the vehicle to my friend would be just as prudent (and would eliminate the issue). But then again, it seems to me that insurance is something you get not because something is probable but because something is possible.