The potential for mobile technology to support bespoke learning activities seamlessly across learning contexts has not been fully realized. We contribute insights gained from four months of field studies of place-based mobile learning in two different contexts: formal education with a primary school and informal, community-led learning with volunteers in a nearby park. For these studies we introduced ParkLearn: a platform for creating, sharing and engaging with place-based mobile learning activities through seamless learning experiences. The platform enables the creation of easily configurable learning activities that leverage the targeted learning environment and mobile devices' hardware to support situated learning. Learners' uploaded responses to activities can be viewed and shared via a website, supporting seamless follow-up classroom activities. By supporting creativity and independence for both learners and activity designers, ParkLearn promoted a sense of ownership, increased engagement in follow-up activities and supported the leveraging of physical and social communal learning resources.