"The Rock" at Horseshoe Bay, near Hovland, MN. Assuming your car has more than a few inches of clearance (unlike mine), you'll be able to drive within a few hundred feet of GZ. Save any information you need before you get too close, because there isn't any cell service for a good amount of miles.
When the water level is low, you can keep your feet dry while rock hopping. It also helps immensely if Superior is being calm. Just be sure to never underestimate the power of wet rocks to totally throw you off. I personally misjudged both while hiding this one.
Please replace as well as or better than you found it, as this area is frequented by muggles. As always, have fun and stay safe.