Treasure Hunt

Yesterday my son decided to start digging in the dirt next to our patio. He has been fascinated with this patch of dirt since we moved in a few years ago. (I think all boys have a natural attraction to digging in the dirt that supersedes all other functions) I even went the extra mile last year and built a 4 x 8 foot sand box for the boys to play in but for reasons I don't quite understand my older son still loves that patch of dirt. Yesterday however, he started a new game where he drew an X into the ground and declared that this was where the treasure was hidden and he needed help to dig it up. I am all for indulging my sons creative side but I cringed at the idea of having a hole next to our patio so I put off digging until it was time for him to go to school. He hasn't mentioned it since but I know this will probably come up again. (my son has a steel trap memory and can remember things that happened two years ago like it was yesterday.)
So I decided that in a few weeks I would bury something in the backyard for them to find. I haven't yet decided what to bury but I am going to draw a treasure map and we will follow the map to a strategically placed X. What to bury is the question. Obviously it has to be something that wont be ruined by being placed in the dirt. A box can be used but I still need to be aware that dirt is not good for a lot of things. And there should probably be pieces of treasure in the ground since I have two sons. Although I have been making headway into teaching them to share, putting two things in the ground reduces the chance of having issues. Also, I have been debating on buying something new or burying an older toy. Right now an older toy is more desirable since I don't want them to have the impression that there is a toy store buried in the back yard.
Personally, I think that this will be fun for them. Not only will it help there imaginations by pretending we are pirates or treasure hunters but the map will make them follow basic directions. The only conditions that I have to remember to tell them is that this is a game that they can only play with myself or my wife. Otherwise I can expect to walk out into the back yard and find a landscape that looks like the moon. Treasure hunt is a fun game, but having try and avoid dozens of small holes can be just a little annoying.