The COVID-19 pandemic has led to a global health crisis and a significant increase in psychological distress and psychopathological symptoms. We conducted a randomized controlled trial with two online interventions derived from positive psychology (PP) and acceptance and commitment therapy (ACT) in N = 138 "homestayers" during the first lock-down period in Germany. PP exercises had a positive impact on anxiety scores, which decreased significantly during the intervention, particularly in participants without access to a garden. Direct or indirect social contact, movement/exercise, and exposure to nature were the most frequently reported helpful coping strategies. We conclude that low-threshold online interventions with a focus on positive aspects of people's lives may be helpful to support mental health during pandemic crises. From a mental health perspective, the results also reinforce the approach of countries that allow their citizens to, for example, still go for walks during periods of contact restrictions, compared to full lock-down conditions.