Sorry to burst your bubble Aloyscious, but the whole purpose of the exercise in preparing a speech is to test your ability to handle things under pressure, without preparation.
Whilst I'm sure your intentions are purely for practice purposes, understand that the best preparation for this is to test yourself. Ask yourself (strange as it might sound) to come up with information on a topic that you have no background in.
Ansett Australia did similar exercises with its' crew in time past. It would not be unusual for potential applicants to be asked "So...tell me about gravy". And for a few minutes, potential applicants had to hold it together to come up with as much CONVERSATION on the topic of gravy.
As far as I'm aware, they're not testing your general knowledge - they're testing your ability to handle questions 'out of left field', as will be asked of you by passengers when you least expect it.
If you look flustered, get angry, upset, etc...chances are you'll be eliminated! If you can handle the stress with style and grace, chances are you'll succeed!