I made this for a friends birthday. Its really quite simple but it takes a while since you need to freeze it layer by layer.
To start you fill the bottom with a few trinkets (I used 3 bouncy balls) put some water in, freeze it. Repeat with other trinkets and layers. I used some of those sticky slap hand toys.
Its fun to make it into a game, make them try and dig each treasure out. I gave him the option to use body heat, salt, and a cup of warm water. Although I'd suggust something to try and dig them out, like a small spoon.