A секретик (секрет=secret, the “ик” part just makes it cute sounding like saying kitty instead of cat) is I guess what you can call a game. Similar to a time capsule, children would collect some random things like flower petals, shiny wrappers from candy, whatever and make a composition out of it. This ‘collage’ would be buried in soil, not too deep though because the point is so that it can be seen. It would then be covered by a piece of broken glass on top. It seems like pointless activity but it’s fun, especially for strangers who find someone’s else секретик and get to enjoy this surprise of a small piece of art by an unknown artist. It’s much like the idea of having something be preserved when the thin ice comes and you can still see grass under it.
Here’s another http://i.piccy.info/i3/b1/50/5f0718f8092f6f500b8993470608.jpeg