I started with a box my grandmother gave me - a carved wooden piece probably not worth much but it looks cool and has a cool latch. Inside I keep things I found outside that are interesting:
Feathers, eggshells, dried blossoms, sweet smelling bark curlicues, seeds and grains and pods...
It's nice to have a "reserve" in case you need something natural to add to your decor. :)
I just added a new piece to my collection last week. Hopefully this doesn't scare you but I thought it was totally fascinating. My cat caught a mole (yayyyyyy!!!) and left in the yard (not so much yayyyy) the bugs did their circle of life thing and pretty soon all that was left was bones and hair - are you freaked out yet? - anyway, I loved biology in school so I noticed that all the gory bits were gone and decided to investigate. This is what I thought was so fascinating. I always knew that moles don't have eyes since they are underground and that would just be problematic for them, but neither do their skulls have eye sockets. Isn't that neat? That clinched it, I'm a nerd.
In any case a treasure box is a nice, tactile way for kids to learn and adults to enjoy the simple, natural treasures that surround us everyday. Happy treasure hunting!