Pick-me-up exercises for after-lunch brain death
Some conferences actually have "nap time" right after lunch. Let people nap if they like or catch up on outside tasks. Such conferences are usually residential, of course, so a nap is a real option. 

A lecture immediately after lunch is going to lose a lot of people, as you say. Your teaching should account for human nature of course. But an active learning session - say role playing an algorithm - might work. If you have to hit them hard for the complete two days (no naps, no games), make sure that it isn't all just lecture and powerpoint. They will be too passive and will zone out too often. Break it up: lecture, discussion, exercises, role-play, feedback, pairing, group work. 

Even a discussion session after lunch would be better than a lecture. You can pose a question to the class about some related topic, preferably one that might generate some differences of view. It could even be somewhat orthogonal to the topic of the course. If the course is about programming, make the question about software process. A simple strategy to start might be to elicit their experiences in development. 

You don't win, nor do they, if you lecture beyond their point of caring.