These can be done so many different ways they are perfect crafts for whatever you have already at home. Personally, if you can, I think little wooden boxes work best but anything from a small cardboard box, little tupperware container, recycled plastic containers, etc will work. I buy lots of little plastic tubs/boxes etc from Dollar Tree so we used one of those. If it's got a lid, it's good!
The purpose is simply letting the child decorate it themselves, they make it special. It's their creativity that makes it shine. I prepared K's box by putting a few layers of brown paint just around the box itself, this gave him an even background to put designs on but left the box covered enough that it's contents won't be revealed. (The orginal box was clear plastic). Two coats of acrylic craft paint did the trick The lid was opaque so I left that for K to decorate.
If you don't have craft paint, glue or tape paper around the "box" and have kids decorate the paper. We began with craft paint, once K had painted the lid, I used hot glue to glue his "gems" or "jewels" to the outside of his box. He showed me where he wanted them placed, (I bought these "gems" at Walmart, but you can find them at Dollar Tree or craft stores). Then he used glitter glue to add further embellishment to the lid. He didn't want to paint the sides of his box, but he did decide to have more gems around it. I used glitter glue to add his name and we're done
If you're low on supplies, paper and crayons around the box will work just fine. If you don't have "gems" go on a treasure hunt for pretty rocks and (with adult oversight or total assistance) super glue or hot glue (or wood glue, gorilla glue- any strong glue- just make sure small children don't have access to it) them to the outside of the box.
*COMING SOON TO EXPLORE AND EXPIREMENT: Kai (husband) and I are going to fill the treasure box with goodies, shells, coins, shiny stuff, and place the treasure box is a protective container and bury it with "X" marking the spot. Then we'll use our treasure map to find our "booty".