Hi Michael,
I teach 6 hour evening courses that are theory in content. What I do is to move the students around, meaning we do small groups, team work, etc.. The process of moving from one part of the room to another is enough to get them refreshed and thinking about the next part of the course. So it doesn't have to be anything major just a "cognitive break" that lets their mind relax a bit and then reengage in the learning mode.
Gary
