I want to thank all who replied and I will take all your comments seriously. I would love to talk about Stone duality and such but I don't think many of our undergrads have ever heard of a topological space. They have heard of topology of course, but mostly they think it concerns things like toruses and Klein bottles. So they know nothing of the point set underpinnings of algebraic topology. My last term before retirement I taught a course called topology and spent exactly 6 lectures on point-set theory (taking a beeline to the Tychonoff theorem) before introducing pi_1 and covering spaces. The students were last year undergrads and a couple of grad students. Do they know what a boolean algebra is? Probably some do, some don't. Groups and abelian groups they will know about, probably modules, etc. Vector space duality is a familiar example, for finite dimension at least. Hi-tech whiteboards and even video-taping are out. I don't think we have any of the former and the one case that I know of a lecture that was video-taped (a fascinating lecture by Conway in the early '70s in which he showed how the game of Life allowed the simulation of self-reproducing Turing-power automata) seems to have disappeared without a trace. I will probably use a blackboard (or greenboard) and chalk, my favorite medium. One suggestion that does appeal is to start with universal mapping properties to explain products and sums. One thing that always struck me was Bill Lawvere's observation that the dual of the usual definition of function as a subset of a product s.t.... namely as a quotient of a sum s.t.... actually corresponds closely to the usual picture we draw when we introduce functions for the first time. I guess I could do worse than build the whole lecture around universal mapping properties. I could mention the somewhat unmotivated definition of (infinite) product of topological spaces as a perfect example of the universal viewpoint. Especially as topologists had come up with that definition independent of category theory. Michael