People interested in the same project/problem can hang out in a space together for 60-90 minute blocks. Somebody near to the project/problem can coordinate discussion, set up hackers, tackle a challenge session topic, etc. Larger groups will get separate space, smaller groups can just spread out in the larger conference room space.
If you have an idea for a breakout topic, you can add it here. (See below the fold for the current list of suggestions.)