A grid of available classrooms are posted, any you put your topic on a sticky and put it on the board. The dots show how many people are interested, and can earn you a larger room.
I took the Teen Maker Arduino and Raspberry Pi gear to let people get a chance to learn about them. There was a group who spent nearly two hours working on various projects.
In the afternoon I gave a session on programming in Python that was attended by a dozen or so.
For myself, I attended an interesting session on web comics. Annie and I also tested some paper airplane designs from the fourth floor balcony. Maybe that can be a session topic for us to teach next time!