Pete Boettke at The Austrian Economists suggests there are just a few important lessons freshman economic students need to be taught. "Focus on first principles (opportunity costs and spontaneous order), and the interpretation of a wide variety of human relationships that can best be explained with the use of these principles" he suggests, " and you will make the subject matter of economics come alive and be exciting to students and leave them wanting more. "
He has two book recommendations which follow this menu: The Economic Way of Thinking by Paul Heyne, which explicitly promotes the above view, and Tim Harford's The Undercover Economist. "The persistent and consistent applications of opportunity cost reasoning and explaining how order emerges out of the behavior of individuals even though it is not anyone's intention to promote the overall order is revealed throughout The Undercover Economist in a vareity of illustrative stories from throughout the developed and developing world."
Personally, Henry Hazlitt's Economics in One Lesson (complete book online here in PDF) would be my recommendation, but then I haven't yet read the other two.