Hmm, I remember back in 3rd form having to do a speech on 'How to ...'. You could go for something almost funny, and deftly reverse the topic, making it into a 'How not to...' (or if they're really stingy, you could say 'How to not...', but still do the same sort of thing).
Here's a couple of (weak) possibles though:
How not to toast a piece of bread (ie- sticking knives in, leave it on too long and start a fire...)
How not to protect valuable documents...
How not to drive a car...
Just try to find something funny (especially if it doesn't really matter all that much), and try to include movements (like acting it out) as well. You may look a fool, but at least it should be funny!