When I read the title of your post, I was ready with a one-word answer, "Clickers." You're already using them, so I'll have to think of something else.
I remember being one of those bodies in large lecture halls for first-year chemistry and physics courses. I was a serious student, always attending the lectures and trying hard to keep focused on the material. The most engaging moments for me seem to be characterized by one or two things:
1. Demonstrations: I always enjoyed seeing demonstrations that worked seamlessly with the material, especially is something popped, smoked, or changed color. These were the days before document cameras and video projectors, but the lecturers sometimes made very good use of what they had, such as using an overhead project to allow everyone to see how a particular chemical reaction lead to a dramatic color change in a liquid. These demonstrations probably required significant preparation if they were to work; I remember some duds, too, because things didn't go as planned.
2. Real world examples: I also remember enjoying having concepts (e.g. viscosity) illustrated through examples that I would encounter in life rather than in the textbook.
I strongly recommend doing a simple google search with these terms: active learning PowerPoint. You'll find all kinds of resources.
Cornell's Center for Teaching Excellence has a good, brief document that includes a brief list of other useful sources:
Bart, M. (2009). PowerPoint: Going beyond bulleted lists. Faculty Focus. Retrieved from: http://www.facultyfocus.com/articles/effective-teaching-strategies/powerpoint-going-beyond-bulleted-lists/
Clark, J. (2008). PowerPoint and pedagogy: Maintaining student interest in university lectures” College Teaching, 56, p. 39-45.
Gier, V.S. & Kreiner, D.S. (2009). Incorporating active learning with PowerPoint-based lectures using content-based questions. Teaching of Psychology, 36, 134–139. University of Minnesota Center for Teaching and Learning (2010).
Active learning with PowerPoint. Retrieved from: http://www1.umn.edu/ohr/teachlearn/tutorials/powerpoint/index.html