The biggest dilemma I come across when planning out a course in economics is keeping students interested. You can only talk about scarce resources, and incentives for so long.
A good example of getting across the idea of scarce resources is to talk with the class about how they came to University that morning. Here, most of the students drive, but I ride my bike. We then start a discussion about why certain choices were made, why they didn't walk or take the bus. The most likely answer is that driving saves time, and the cost of gas and parking is more than offset by the time and inconvenience of walking or taking the bus.