Q&A sessions after talks and workshops seem engaging, and they are somewhat. The highest level of engagement is actual Q&A where everything begins with people's questions, followed by talking shaped by the emergent questions.
Business as usual is A&Q where someone produces answers to their own questions, not to the group's questions. This is the least engaging model and easy to correct. The downside is that the agenda is emergent rather than prescribed. This wierds people who have never been exposed to such a thing. The payoff is it always works and it dramatically increases interest and retention.