Student teachers

Give small groups of students a problem to solve or a questions to answer.  Allow time for discussion, depending on the difficulty or complexity of the questions, and then have each group "teach" their problem to the rest of the class.  They should present the question and then explain their process for solving it in addition to the answer they came up with.  It's nice for the rest of the students to hear the process from someone other than the teacher, and the process of explaining to the rest of the class helps the student teachers solidify their understanding of the problem.