I really like the idea, and applaud the community spirit…
But why should you have to come up with a new prize and traipse around the country finding new interesting places to hide goodies for strangers OP?
The “prize” is almost irrelevant IMO, being the first to navigate to the location is surely the primary reward, you get to show off your “outdoorsmanship” in a non-stravaey way…
So why not run it on a “pay it on” type basis, getting there first and claiming the trophy means you have to come up with the next spot in a month’s time, and it has to be a minimum distance from the previous spot, say 120 miles? Meaning the whole thing (hopefully) migrates round the country of its own accord, and doesn’t just rely on one person to keep on going…