Twenty minutes to demonstrate binary sort by tearing apart (literally!) phone books to find a person listed there, is how CS50 opens its classes. Take a look at the opencourseware site cs50.tv. It's practical, it's interactive, and it really shows the computational aspects that we take for granted. Twenty minutes to demonstrate selection sort and merge sort might be a bit tight though.
I think a discussion of the more "non-computer" parts of computer science would keep an audience more interested than a discussion about programming languages, which could easily lose people in the first five minutes.