This is a very simple idea. It's called the mystery box. You can see a photo of ours below, although it can be a simple as a cardboard box with a hole cut in it. It is excellent to use as a starter for circle time. You can place an object inside relating to your topic and have the children try to guess what it is. There are so many ways you can incorporate this in your classroom. Another example is put cut-out foam letters inside and for small group, have the child try to guess what letter they pick up. Then you can have them match their letter to an object that begins with that sound. Whatever you do with it, the children will get excited because of the element of mystery surrounding it. The want to know what's in it so bad, they will try to pull it out, get just a little peek, or look around the back if yours is clear on the back like mine!
For an extension, after you have done it a few times with the kids so they have the hang of it, have one child choose an item to put in and allow the other children to guess. Try things with different textures, so they will have to think about it. You could even put 2 different things in and describe one to see if they can pull out the correct one. Allow them to shake the box so they can see what it sounds to help them if they can't guess at first.