During CERME10 in Dublin, a collaboration between the University of Turin and the University of Frankfurt has been initiated. The outcome was a module of a Massive Open Online Course (MOOC) for in-service teachers to improve mathematics teaching by creating outdoor math trail tasks for their students with the help of MathCityMap (MCM). MCM is a digital tool to create, manage and run math trails with mobile devices. Teachers had to create math trail tasks that were in line with previously defined task design guidelines to complete the MOOC module. An expert review system was used to improve and ensure material quality. The involvement of the teachers, the efforts of the reviewers and, in general, this collaboration have given satisfactory results that will be presented in this paper.