## 1 Introduction  
It is a fact that overall life expectancy in developed countries is constantly increasing. According to the World Health Organization (WHO), populations around the world are rapidly aging [1]. The aging of the population has personal and social direct implications, as it affects and has an impact on issues, such as family organization, pension systems, health systems, etc. To address the challenges of these transformations, different sectors are working to re-define the concept of old age in order to adapt it to the new political, social, economic and cultural space resulting from the increase of older people. This is the context where the philosophy of active aging appears, a term adopted at the end of the 90's by the WHO as a re-formulation of what it means to be old in today's society from the perspective of the rights of the elderly. The WHO also identifies strategies for people to successfully address this stage of their lives. Among the different aspects identified as positive for aging actively, we can point out the following ones according to the interests of this research: The benefits of taking physical exercise, the benefits of nature and outdoor activities and the benefits of continuous learning and the acquisition of skills, especially in Information and Communications Technology (ICT). This approach completely transforms the social and personal perception of old age and debunks the myth that at this age it is too late to adopt a healthy lifestyle, to go on learning and to have an active social life. A plethora of research has proven that regular physical activity contributes to maintaining health in older adults providing physiological, psychological, and cognitive benefits [2] that can be both preventive and recuperative. For this reason, this article does not explore the quantification of the benefits of exercise in health. Besides, being exposed to nature and spending time outdoors has also beneficial effects for health; years of study have proven that nature offers physical, social and psychological benefits that range from reducing mortality, due to cardiovascular diseases to improving one's own sense of well-being and cognitive performance [3; 4]. As a result of these effects, evidence starts to emerge suggesting that physical activity in outdoor natural environments provides benefits that go beyond what is expected from the same activity performed in gyms or indoor spaces [5; 6]. However, the majority of older people have sedentary lifestyles and habits. According to the WHO, in 2016, 39% of adults over 18 were overweight, and 13% were obese [7]. Among the elderly, there is a high reluctance to physical activity, due to the lack of interest and access, and also to motor impairments, that are common on these ages. A number of environmental barriers have also been identified that make it difficult for older people to take exercise outdoors, such as the weather or insecurity, but also the perception of the elderly on their physical abilities and the conditions of outdoor spaces [8]. Therefore, it is necessary to boost initiatives and projects that make physical exercise attainable for older people, providing tools and scenarios where physical activity and social/cognitive development are combined. In this article, we introduce HybridPLAY, a self-developed technology that, from the field of video games and audio-visual entertainment, is aimed at facilitating outdoor physical activity by introducing a system that transforms Outdoor Fitness Equipment (OFE) in interfaces for interactive digital game, favouring in this way motor activity and social relations in an outdoor natural environment. HybridPLAY consists of a sensing device that can be attached to OFEs and that is connected with a mobile phone that contains a set of mini-games. One of the aims of HybridPLAY is to provide a collaborative environment that it is established with verbal communication between a user handling the mobile device and other user doing exercise with the OFE.