Bring something you're working on and want to chat about, or just bring an inquisitive disposition. For now we're sticking with an unstructured salon (http://en.wikipedia.org/wiki/Salon_%28gathering%29) approach.
Oleg suggested last time that we introduce a problem at our next meeting and then we can share and discuss solutions at the following meeting. I thought that sounded like an entertaining idea. He had some thoughts on big-data problems that would be interesting to pursue, but I think we're open to other suggestions as well.