I had to teach cross-country planning for my CFI. I had no idea what I was going to have to teach going in, so I didn't have a flight plan ready. He said, "Teach me how to create a flight plan to go to Ryan Field in Tucson." I pulled out my chart and flight plan log and went step-by-step and made a flight plan.
He didn't expect me to spend 4 hours and get into every detail, he just wanted to know that I had the basics down and I knew what I was doing. I went through choosing a path, taking into account that I would either go around or through the PHX Bravo and considering airspaces I'd encounter. I went through choosing good checkpoints, determining true course and converting that to magnetic heading, how to use an E6B, reading performance charts to determine top of climb, fuel burn, time enroute, etc.
Basically, be able to explain everything you did on your flight plan and the reasons behind choices you made. I think it would be best to have a flight plan ready, then create the same flight plan in front of him. Quiz the DE, acting as a student, about decisions he would make. Is that a good checkpoint? What makes it good/bad? Would you contact Approach? If so, when? What frequency? What would you say? What altitude will you fly at and why? etc etc
Teach him wind triangles, some DEs really like that!