Social interactions and active activities are key to psychological health, but how do these activities improve wellbeing? Building on work showing that positive emotional experiences help build psychological resources (Fredrickson, 2001), we test the prediction that momentary positive emotional experiences mediate the relationships between social interactions, active activities, and psychological wellbeing. In a pre-registered experience sampling study, 106 participants reported their activities, momentary emotional experiences, and wellbeing five times per day for 15 days (7,110 observations). Participants reported experiencing more intense positive emotions when interacting with others (especially face-to-face), and when they engaged in active activities. The results confirm our hypothesis: The intensity of positive emotional experiences partially mediated the positive relationships between social interactions and wellbeing and between active activities and wellbeing. These findings show that momentary emotional experiences can be elicited by social and active activities, helping explain why these activities aid our psychological health.