I think almost any topic in computer science can be used for giving an interesting talk, but some are better suited, the more important part is the presentation.
Fun Side of Computer Science
I have used various games from Combinatorial Games Theory, mainly from Richard Guy's "Fair Games" and Elwyn R. Berlekamp, John H. Conway, and Richard K. Guy's "Winning Ways for your Mathematical Plays" (wiki).
They are fun, and you can play them in the class with them and let them find the right way to play it, give some hints so at the end they find the way to win the game. These games are probably more suitable for younger students.
There are other fun topics in Computer Science where you can pick a problem which is more suitable for your audience and use it to engage them.
Philosophical Side of Computer Science
There many topics in theoretical computer science which are related to philosophy and the big questions. From Gödel's incompleteness theorem to zero-knowledge proofs, security, privacy, algorithmic game theory, P vs NP, machine learning, ... I would not go into details, just demonstrate that the problems are interesting, they are more than just computer science, they are related to big questions. (Take a look at Scott Aaronson's Quantum Computing Since Democritus and Great Ideas In Theoretical Computer Science lectures). Don't make them feel like the topic is dead (i.e. all questions are answered), make them feel that the area is alive, there has been progress but there are still big challenges ahead, and it is a journey to an undiscovered land.
Technological Side of Computer Science
Talk about the computer science behind technologies. There are so many topic that one can choose here, familiar technologies from video-games to Google search, machine translation, vision, ... technologies that everyone uses each day, or even unfamiliar ones. Talk about in progress and next generation technologies, about the impact they have had on our lives, and how they have improved it. Talk about research going on in big famous companies (like Google, Microsoft, Apple, IBM, ...) and products they develop. Talk about big problems of our time and what effect computer science has on them.
Mathematical Side of Computer Science
This is good for students who are already interested in mathematics, those interested in the pure and exact side, but without combining it with other theme mentioned above it won't be as effective to other students. I would go with a big question and at some point mention start talking about mathematical problems involved.
Interdisciplinary Side of Computer Science
Computer Science is probably one of the most interdisciplinary subjects, there are some connection with almost any other subject, humanistic (sociology, linguistics, economics, philosophy, ...), natural sciences (mathematics, physics, ...), biology, medical sciences, art, engineering (electronics, mechanics, ...), ... anything! Whatever topic you are interested in, there is something in computer science that is related to it! As Scott said, Every Other Major Sucks By Comparison :).
All of Them
You can also try to mention all of the themes I have mentioned above. I haven't tried this, and I am not sure how effective it would be. You have to transfer the feeling and make the point, and it takes sometime. One other options is to mention all of them briefly at the start (or the end) and then go on with one of them, and tell them that they can contact you to get more informations about the other ones if they are interested.
Whatever you are going to talk about, you should be enthusiastic about it. It is going to be much more difficult to interest them in a topic which is not really interesting to yourself. Tell them about your own reasons for selecting computer science. And don't be boring.