Here's a story about how my grandpa used to take me fishing and how those memories led to this fun past time: he didn't. I saw a dad do this once while camping and thought it was genius.
Ok, on to the game.
Stuff you need: metal detector, shovel (optional), metal containers (we use altoids cans), treasure that fits in that metal container, park map, pen/pencil, enthusiasm.
It's pretty self explanatory and it's for kids/pre-teens, but there are a few tips that make it more fun. If you can swing it, have one group go ahead early. This will probably be the group with fewer or no kids (bathroom breaks). This group gets the altoids cans, shovel, treasure, and will get park maps. Get there, grab a park map at the gate, set up camp, then go hide the treasure. Make sure you mark it on the park map. If you don't feel comfortable digging, hide the can in a pile of leaves or debris.
The next group has the kids, metal detectors, and enthusiasm. When they get to camp, it's already set up. They get to jump out and start the hunt. You get to sit by the fire and enjoy a tasty beverage. Disclaimer: this is also a game to help hone land navigation skills. Use good judgement if it's appropriate for your kids and their skill level.
Another idea for this game - bring in a scooby doo element. Let them know that one of the marked treasures on the map is actually a mislabeled snipe nest. Tell them you have no clue which one it is, but they should be fine as snipes are generally docile creatures and are responsible for fewer deaths in the US each year than vending machines.