Ruminative thoughts induced by the COVID-19 pandemic have an adverse impact on individual well-being. However, little is known about how to alleviate such a negative effect – let alone how a person can flourish during crises. The current study uses the self-determination model of flow to propose that two proactive behaviors, strength use and playful design, are positively related to the flow experience, which, in turn, is positively related to daily flourishing. Moreover, we propose that the effects of proactive behaviors on flow are stronger when individuals ruminate more about COVID-19 pandemic. Using a day reconstruction method, we collected data from university students across five consecutive days (N-person = 135, N-day = 665, N-activity = 2985). Multilevel results showed that strengths use and playful design were positively related to flow experience. In turn, flow experience was positively related to daily flourishing. Moreover, we found that COVID-19 rumination moderated the associations between proactive behaviors and flow, such that strengths use and playful design were more effective to foster flow when individuals had a higher (vs. lower) level of rumination. The results imply that in order to function well and flourish during a crisis, individuals could make use of their strengths or playfully design the activities in their daily lives. In a broader sense, we provide concrete behavioral strategies to cope with the downsides of negative events.