Some ideas:
A different person could be in charge of each meeting - they might pick out an article and tell everyone to read it before that meeting.
One question is whether you want to focus your attention broadly on lots of skills & topics or narrowly on mastering a small number of skills or topics.
You could also approach one topic from many angles. For instance, with happiness you could first read some articles on the psychology of happiness (or a book like Jon Haidt's). Then have a meeting where you talk about happiness in your own lives: what makes you happy or unhappy, things you try to do to be happier, ways you might be irrational about pursuing your own happiness, changes you could make. Then each come up with a plan for improving your own happiness, and discuss & refine those plans so you're ready to carry them out. Then each try out those plans, and occasionally check in on how they're going and do group problem solving if they run into snags or you're ready to move on to a further plan.
Last night, here in Portland (OR), some friends and I got together to try to start Rationality Dojo. We talked about it for a while and came up with exactly 4 exercises that we could readily practice:
We also had a whole bunch of semi-formed ideas about selecting a target (happiness, health) and optimizing it a month at a time. Starting a dojo, in a time before organized martial arts, was surely incredibly difficult. I hope we can accrete exercises rather than require a single sensei to invent the majority of the discipline. So I've added a category to the wiki, and I'm asking here. Do you have ideas or refinements for exercises to fit within rationality dojo?