I always sing this song when I use the magic box:

¡En la caja mágica encontrarás
Una sorpresa que te encantará! (3 times)
Click here to sample/buy song on Amazon

We also chant:
¿Qué hay, qué hay en la caja mágica?
¿Qué hay, qué hay en la caja mágica?

After singing the song,  I will also give them clues. I usually put something related to what we have been doing in class in the box. For example,

1. Hay un animal.
2. Hay un animal verde.
3. Hay un animal verde que tiene ojos grandes.
4. Hay un animal verde que tiene ojos grandes y salta.

Then I count to three and start taking guesses. Once I reveal the the secret, I ask questions about it again and use a lot of gestures. These are some of the questions I would ask:

1. ¿Es un animal o una persona? ¿Qué es? ¿Qué animal es?
2. ¿De qué color es? ¿Es verde o amarillo?
3. ¿Tiene ojos grandes o pequeños?
4. ¿Nada o salta?
5. ¿Te gustan las ranas?

I really love this teaching strategy because it gets the children engaged.  I now have two cajas mágicas.

Please see the pictures below. In the first picture, you see a shoe box that I open to reveal “el secreto.”
The box in the second picture is “la caja mágica sensorial.” I place objects in them, and children can take a guess by telling me “es grande,” “es pequeño,” “es suave,” “es duro,” and so on. If the children say the words in English, for example by saying “it’s soft,” I would say “sí, es suave,” “¿clase, es suave?” Get ready for some noise and for everyone to expect (or demand!) a turn.

Have fun teaching!
Carolina