While deciding upon a data gathering activity, I ended up creating an activity which I have named: the quadrants for lessons learned.
Please find below picture depicting the activity. It has two axis: success versus failure, and planned versus unplanned. It has 4 quadrants: successful and planned, planned but failed, Unintended but successful, and Failed and not Planned.
The photo below is a result of this activity. As you can see, it generated lots of insights on lessons learned. The participants enjoyed this activity. It gave them lots of data points to talk about. I personally liked it because it opened our eyes for interesting quadrants, such as the accidental success or failure despite of planning.
After gathering the notes on the white boards, I asked each participant to use up to 5 dots on the cards he/she cares the most.