Help ( web/teaching related)

im doing a teaching course and next week I have to do a 20 min teaching session to the rest of my group teaching them something from my field ( webdesign/design)

So I’m gonna talk about basic web design principals and I was going to look at Navigation, way finding on a site, text for web ( structuring, colour, formats)

I was wondering if anyone could suggest anything else in the web design area I could teach. I can only use a white board and hand outs as teaching aids so I can’t get too technical ( no laptops, projectors ect)

any help would be awesome