If you want them to listen to everything you say, you will have to reinforce your speaking/oratory skills. If instead you want them to learn, give them something to think about and play with while you are talking. Have scratch paper and pencils and encourage their use. Alternatively, have one or two (not more than two) items to use as a visual focus as well as a subject for the talk.

As an example, I had to TA a section that involved teaching (something like) the change of variable technique for multiple integrals. I took them to a place on campus that had a hemispherical skylight at eye level with some gridlines on it, where you could see the shadow of the lines on the floor below. I used this as an example of how one needs to be concerned about how volume elements change as you try to approximate the integral, and why you needed the determinant to represent this change.

If you were to do something similar (say Leonardo da Vinci's observation about how flow at the mouth of the river was the sum of the flows at the branches), you might invoke a tree or idealization thereof to get your point across. Also, don't expect to have the aid carry the audience through the talk for more than about 10 minutes on a single topic. You can reuse it for more than one topic though.

Gerhard "Ask Me About Minimalist Environments" Paseman, 2011.09.14