Having time for private reflection AND time to interact w/ other group members appears to increase creative output over either approach in isolation.
I think key to this is that group members feel comfortable to share their ideas, good and bad. Increase that comfort level, and I bet you'd increase the cognitive diversity of the group. Bad ideas are often stepping stones to better ideas, but we don't get those stepping stones if people are afraid to share. Perhaps making it a game of group interaction: Give a prize for worst idea, and more of those
"bad" ideas will at least be shared w/ group and become the seeds to other ideas. </rant>