Musical instruments have a property of long-term engagement: people frequently become so engaged with them that they practice and play them for years, despite receiving no compensation other than enjoyment. We examine this phenomenon by analysing how the intrinsic motives mastery, autonomy, and purpose are built into the design of musical instruments; because, according to the self-determination theory of motivation, these three motives impact whether an activity might be found enjoyable. This analysis resulted in the identification of seven abstract qualities, inherent to the activity of music making or to the design of musical instruments, which contribute to the three intrinsic motives. These seven qualities can be treated as heuristics for the design of human-computer interfaces that have long-term engagement. These heuristics can be used throughout the design process, from the preliminary stage of idea generation to the evaluation stage of finished prototypes. Interfaces with instrument-like long-term engagement would be useful in many applications, both inside and outside the realm of music: they seem particularly suited for applications based on the attainment of long-term goals, which can be found in fields such as physical fitness, rehabilitation, education, and many others. In this chapter, we discuss an interface prototype we created and its pending evaluation. This interface, a rehabilitative rhythm game, serves as a case study showing how the heuristics might be used during the design process.