Let's go back to the basics. A common definition for "mainstream" is:
The ideas, attitudes, or activities that are regarded as normal or conventional; the dominant trend in opinionIt seems that, contrary to Mankiw's argument about the utility of the building blocks, mainstream economics is more appropriately defended as a convention or "common language". This poses a slight problem -- for understanding the "power" and prevalence of mainstream economic analysis, certainly -- but also for thinking about how to take a different approach in the classroom. As the reader might expect from this definition, teaching parts of economic thought considered outside the mainstream, ie. ideas that may be regarded as abnormal or unconventional or not the dominant opinion, can be a daunting task, particularly when attempted at the 'principles' level of economics.
Many principles courses, micro or macro, often provide perverse incentives to teach - and to learn - solely mainstream material. Consider the teacher's perspective. He or she would like to do what they think is best for their students by teaching broadly and discussing economic history and different schools of thought and how their assumptions may be different, etc. Alas, textbooks to teach non-mainstream ideas at an introductory level are not very visible. Mankiw of course has the largest market share of introductory textbooks and teachers often receive free complimentary versions for review not just from Mankiw but from a swath of other mainstream textbook authors - and department heads often encourage the use of these mainstream texts.
But it's more than encouragement. The final exam in many principles courses, partly due to their size, are departmental multiple choice exams. This means that the well-meaning pluralist teacher now has to dedicate most of his or her time teaching to the (mainstream) test or else risk students' grades suffering the consequences. Because teachers are in large part evaluated based on their trending performance every semester relative to other teachers, the incentive is large to stick to the mainstream text throughout the semester.
But what about from the student's perspective? Much like the teacher, their performance matters too. Firstly, if they are in an honors program they often have to maintain a C or above, and even students who want to learn the heterodox topics a teacher may teach, does so at risk of a lower grade. Here again, the final exam, and indeed perhaps other quizzes and homework in the case where the teacher may not cover pluralist topics for reasons stated above, are often geared toward the mainstream, and since study time is a finite resource (of which hopefully all economists can agree that there are only 24 hours in a day), the student has the choice to focus effort on mainstream study, or shift some of that precious time to studying heterodox material that may not benefit their grade. The institution is set up in such a way so that the student has incentive to choose the former.
Additionally, most students who take introductory econ courses are not economics majors. Psychology, business, accounting, sociology, and political science majors take the course as well. A more pluralist economics discourse would account for their own major's contributions to economics, which is often ignored or brushed aside in mainstream texts. Thus, when such a student encounters their mainstream economics course, while try as they may to stay engaged, they are often found napping as the teacher goes over the concepts of utility maximization or preference ordering, etc. This does not help anyone - student, teacher, or department.
Nevertheless, all is not lost. There are some textbooks that do exist that try to incorporate pluralist economics at an introductory or intermediate level. One of the best set of resources is courtesy Tufts University's Global Development and Environment Institute. They have recently (2008) published both micro and macro textbooks that speak not only to mainstream concepts but also to issues of environmental economics, serious discussion at both micro and macro level of income inequality, institutional and historical trends in the US macro-economy, non-mainstream discussion of international trade, and alternative measures of economic well-being and growth. Best of all, their supplemental materials and significant portions of their texts are freely available on their website. And, the published textbooks are affordable at only $50 each. Another great intro text, the Economics Anti-Textbook serves to pick apart the assumption of mainstream economic theory - exposing what are often taught as simplifying assumptions for what they really are - critical assumptions. Another great macro text by Steve Cohn, Macroeconomics: A Critical Approach, serves to discuss mainstream theory but expands the discussion to how financial markets really work, financial instability, Marxist contributions to economics, and so on.
A more complete list of texts and pluralist publications in general can be found here. While there are certainly institutional barriers to teaching a more pluralist kind of economics, and while there are certainly financial and other barriers to assigning multiple texts to students, using materials or pieces from some of these texts can at a minimum, enhance a student's understanding of economics in its many schools of thought.
Beyond textbooks, there is a plethora of non-conventional tools that a pluralist teacher could use to engage students. Many economics teachers invite other professors within economics as guest lecturers. Some teachers may even branch out of economics and occasionally invite someone in a Business Finance field to lead a group discussion. But we would venture to guess few mainstream teachers really embrace plurality enough to invite, for example, an I/O psychologist to discuss motivations for decision making in labor markets that may not be deemed to be 'rational'.
There are other tools a teacher who wants to expand the usual economics dialog can utilize. How about adding some behavioral economics to the classroom by running the occasional group 'experiment'? One of the best creators of such experiments is Prof. Denise Hazlett of Whitman College: experiments that show how the normal loan-able funds model can breakdown in times of economic and inflation uncertainty, or how investment coordination failures can result in large recessions.
Finally, we suggest mixing the typical lecture up with topics that go beyond the normal mainstream discussion by using a good podcast (radio is not dead!) or perhaps a YouTube video. These mediums are particularly helpful at engaging a student - especially students from outside the economics department.