Hi Dave,
what about running a build along series.
The idea here is that you pick some thing like a Quadcopter or Delta robot or a CNC machine which you build over a series where each episode requires your audience to follow along. These kind of projects are fun, especially if you can keep the monthly costs below $100.00 (This is normally acceptable for the Finance Director in my house ;-).
Something that would not only be educational, but useful afterwards.
Just a thought.