Reptiles and invertebrates are not well known but fears of them are one of the largest in the country. Why not do a "sponsored hold" of a tarantula or a 10ft burmese python, live on air. Let the listeners listen to the initial fear and uncertainty and then hear the realisation that its not as bad as you thought!
Education is the most valuable tool in over coming irrational fears and hopefully this would be one way to do that. Educate :)