I often use a "mystery box" to introduce a new unit or concept to my students. For example, when we start a unit on Maps, I put a map in the box, and the students have to ask me questions to find out what's in the box. It's sort of like 20 questions, but I don't always limit the number of questions to 20. The directions I give to the students for this activity are:
1. The questions they ask me must have a yes or no answer, like - Do you use it inside a house?
2. They must listen to each others' questions and answers to find out what's in the box.
3. I suggest they try to find out properties of the item, rather than just guessing about it being specific items like - Is it a pencil?
4. The person who guesses correctly gets to keep the item on their desk for the rest of the day.
At times, the students have become frustrated, and I remember someone asking once if I would just tell them what it was, but I always review what they've learned so far from the answers to the questions. And you might want to "veer" them in the right direction with a little clue if needed.
I've put a thermometer in the box when beginning a study on weather, or chopsticks when studying Japan - anything with a link to the unit would be suitable. The students are very engaged and interested with this kind of kick-off to a unit.
Anyway - here is how my new Mystery Box turned out. I added some letter stickers that I had to the side: