Dominant psychological models of wellbeing neglect the role that nature connection and other key factors, such as positive health behaviours and behaviour change, play in determining wellbeing. The present mixed-methods evaluation explores the impact of ”Surf-Ability”, an adapted surf therapy intervention delivered in collaboration with a UK neurorehabilitation service, on individuals with acquired brain injury (ABI) as part of an effort to design interventions based on advances in wellbeing science. Following five surf-therapy sessions, within-subjects analysis (n= 15) revealed significant improvements on the Warwick–Edinburgh mental wellbeing scale (t (15) = −2.164, p = 0.048), as well as in anxiety and happiness as measured via a brief visual analogue. No significant changes occurred in the Hospital Anxiety and Depression Scale (HADS) or resting heart rate variability (HRV). A ripple effects mapping (REM) session at 6–10 months follow-up (n = 6) revealed that the physical and psychological experience of a nature-based challenge initiated a mindset shift in participants, which ultimately led to them adopting wellbeing-promoting long-term behaviour changes. These changes occurred at the scale of (1) individual wellbeing—increased mindfulness and physical activity; (2) collective wellbeing—improved relationships, community participation and contribution to organisations; and (3) planetary wellbeing—connection to nature. These findings align with the GENIAL theoretical framework, which defines wellbeing from a biopsychosocial ecological perspective across multiple levels of scale. The findings support the need for healthcare providers—including neurorehabilitation services—to enhance interventions for patients by incorporating novel factors that improve wellbeing, such as nature-connection.