I have discussed informal learning before in this post here, however one of the challenges in informal learning is allowing it to happen. The reason it's a challenge is that you cannot control it. The moment you control informal learning, it stops being informal learning. At best you can foster an environment conducive to formal learning.
I recall taking some training some time in my past where the instructor noticed that the group activity that I was participating in had completed their work and we began discussing a topic related to the lessons we were involved in. In a move that was more police-like rather than trainer-like, she interrupted what we were discussing and told us to return to our original seats if we were finished the group activity.
I was surprised that we were shut down. The conversation which our group continued during our lunch break was very informative and I felt that the collaborative nature that this group dynamic created was very beneficial to those that were involved. When I returned to work after the training and asked by my manager if I had learned anything, my response was yes, but I almost didn't.
Make sure you allow time to let informal learning happen. It's very collaborative and social and can happen when you least expect it. Because it's driven by the participants it usually is far more effective then some written curriculum so don't stifle it when it happens.