Active reminiscence is an important aspect of healthy aging and wellbeing. But recalling life stories on the spot is not easy. This paper addresses potential stimulus material in the context of HistoryTelling, an interactive, social platform for older adults on which they can record and pass on life stories and historical events to younger generations. In an HCD+ approach, we conducted workshops and interviews to get information about the occasion to pass on life stories and potential design recommendations on stimulus material. We found that both, the old and the young enjoyed the time spent together, appreciated the possibility to bond within the workshop and that pictures and questions were suitable as stimulus material for reminiscing, but that stories work best. We conclude with an outlook on our plans for further development of the HistoryTelling system.