Access and exposure to public green space might be critical to health promotion and prevention of mental ill health. However, it is uncertain if differential health and mental health benefits are associated with undertaking different activities in public green space. We evaluated the health and wellbeing benefits of different activities in different locations of public green spaces in urban and semi-urban areas. We used a mixed-methods before-and-after design. Volunteers at three conservation sites were recruited and took part in group guided walks, practical conservation tasks or citizen science. Repeated measures one-way ANOVAs with Bonferroni correction assessed the relationship between location and activity type on change in acute subjective mood from pre- to post-activity, measured with the UWIST Mood Adjective Checklist (UWIST-MACL). Qualitative semi-structured interviews were undertaken and analysed thematically to explore participants’ perceptions about the health and wellbeing benefits of activities in public green space. Forty-five participants were recruited, leading to 65 independent observations. Walking, conservation and citizen science in public green space were associated with improved mood. Across all participants acute subjective mood improved across all domains of the UWIST-MACL. There was a significant association between reduction in stress and location (p = 0.009). Qualitatively participants reported that conservation and citizen science conferred co-benefits to the environment and individual health and well-being and were perceived as purposeful. Undertaking purposeful activity in public green space has the potential to promote health and prevent mental ill health.