They each take turns carrying the GPS unit, with Sarah constantly announcing the number of feet left and Eric ignoring the beep that indicates when you're close. Each time, Brian has gently taken it from them, walked a little farther, and then stood very close, but not too close, so that they could have the opportunity to find the treasure. We've learned to bring something old to trade for something new, and for a little while, whatever they find is their new favorite thing.
What I love even more than seeing their excitement is the exploring that happens on the way back to the car. (They are far too focused to do this during the first half of the walk.) Logs to balance on, streams to cross (or play in), and crayfish to catch (until they pinch) are treasures of their own, yes?