Designing for open-ended play poses specific new challenges to designers. Designing for closed games includes defining rules and goals to balanced the game properly. A design for open-ended play has no predefined rules and goals. The design needs to provide users with more freedom to continually change goals and rules of play, which distinguishes the field from designs of closed games. Gaining knowledge on the design process of creating this freedom is essential. For this purpose, an integrated model for open-ended play is proposed. This model is based on a combination of two existing models: Hunicke’s Mechanics Dynamics and Aesthetics (MDA) model and Grünvogel’s formal models for game design. Both of the above mentioned existing models are generalized to make them applicable for analyzing open-ended play. In the proposed combined model we distinguish between the perspectives of the design, and the perspective of play. It addresses how to handle changing rules and goals, instead of the assumptions that rules and goals do not change. Furthermore, the model was used to improve our understanding on progression and emergence, two key concepts that are commonly used in game design. The integrated model for open-ended play (IMO) was used in a preliminary case study with a digital play application, an interactive environment for open-ended play named the GlowSteps, to evaluate the model and to underline our insights on emergence and progression.