Background: South African learners face the double disadvantage of living in low socioeconomic conditions with access to few resources and attending schools with challenging learning conditions. Mathematics performance reflects such conditions with extreme performance gaps between wealthier and poorer learners. The need for early intervention is increasingly acknowledged. Aim: In this conceptual article, we draw out the features of mathematics games that can be used to develop learners' foundational number sense that will impact their learning trajectories. Setting: Based on our experience in working with learners from low socio-economic backgrounds in after-school mathematics clubs, we propose that these are opportune spaces for intervention. Methods: Drawing both on the research literature and our experience, we exemplify and discuss key features of mathematics games and argue why these are important to support the development of number sense. Results: We argue that to meaningfully develop young learners' number sense, activities need to be sufficiently engaging to retain their interest and be presented in a manner that club practitioners, who might not be teachers, are able to facilitate. Drawing on our experiences of designing and running mathematics clubs, we discuss why components like built-in checks, readily available resources, connection to calculation strategies and transferability to home settings are important design features. Conclusion: Particular design features are important to enable maths games to support learners' development of efficient and effective calculation strategies. Contribution: This article contributes a framework of key features for effective use of games to develop mental mathematics strategies for number sense.