Get some computers with Notepad++ installed on them and a file that has some various lines of text. Teach them about pattern matching and regular expressions. It doesn't really require any previous knowledge, and it makes you kind of think like a programmer. It's very useful even if you only have a basic knowledge of it, especially when tearing through log files with grep. Some of the students might not find it interesting at all, but I think you'll find that regardless of what you do.