The only things that should be traded in this cache are two things. The first is seashells. (Hence the name) Find awesome ones on the beach and come trade them with the ones in the cache. And remember, the rules still apply that you need to trade equal or greater value, don't take an awesome shell and replace it with a lame one.
The other thing to trade is trackables. It will only hold small ones, but feel free to put them in. and have fun.