I love those Find It Games. I have made a few myself.
You can get some of those small packs of shaped and decorated buttons (they have them at Michaels) to use. They are usually packaged by theme. I just cut off the attachment for sewing on the button so they were flat but you probably wouldn't have to. With Christmas coming you might be able to find lots of tiny stuff to put in there. Maybe you can make a Christmas themed one for the holidays. They make all those tiny mini ornaments, etc... Have fun!