This Activity is meant for Boy Scouts.

3 scouts.
possibly paper and pencil for each patrol

The leader is in on the activity so he cooperates.

Two scouts dress in disquises beforehand. The more wild items like hat, sunglasses, shoes, gloves, fake beard, ... they wear, the better.

At some point during the meeting, the two rush in and demand the leader's wallet.
The leader hands it over, and they run out of the room.

The third scout enters the room and asks to have all patrol leaders meet with him. He explains that, in order to apprehend the thiefs, he needs their patrol to come up with a detailed description of them. There will be a reward for the patrol that is most helpful.
They have 7 minutes to create their list.

Once the lists are turned in, the detective departs to review the lists with the two thiefs and determine a winner.

The third scout marches the two apprehended thiefs back in and announces they were caught, mostly due to the excellent description from the winning patrol.