Give each student three 3X5 cards. Have them write a question on each card that an investigator might ask. The questions should require the use of a SM to answer. (They can look at the missionary application on the back of their SM cards to help them come up with these questions.)
Divide the class into two groups. One group is the Investigators. Give each investigator three mini bags of M&Ms. The other group is the Missionaries (they need their scriptures with them).
On go the missionaries approach an investigator and announce that they are a missionary for the Church of Jesus Christ of Latter Day Saints. The missionary then asks the investigator if she would like to hear a message.
The investigator then pulls out one of her cards and tells the missionary that she has a question. She proceeds to ask the question and the missionary must locate a SM to help answer the question.
If the missionary is successful, then the investigator thanks him for sharing a scripture that answered her question and gives the missionary a bag of M&Ms. (You could say that the investigator does not have to give up the M&Ms unless the missionary introduced himself properly.)
They part and find new partners.
After some time, switch roles.