Hi Larry:
Mixing up activity is a great tool, especially in some classes, math among them. Keeping students focused and attentive is our job and can be handled from simple steps (moving about the room, calling on non-volunteer students, stepping next to a student, etc), to more agressive moves(such as changing activities, taking a break, facilitating a discussion, etc.). Keeping the class interesting will help with reengagement too.
Regards, Barry
