As Mixed Reality (MR) games are becoming more and more integrated into healthcare, the need to create engaging and motivating games for rehabilitation purposes is of the utmost importance. To investigate what factors in MR games make users intrinsically motivated and engaged, 13 participants played five games of different types and rewards. After each game, a questionnaire measuring four subcategories for intrinsic motivation was performed. After playing all games, interviews were conducted to voice the participants' opinions. The results showed that there were multiple factors in an MR game contributing to the engagement and motivation of the users. The questionnaire results also revealed a significant difference between the subcategory interest/enjoyment and perceived competence. The themes identified from the interviews were the need for the game to be sufficiently challenging, interaction with MR, curiosity, as well as emotional design. These conclusions suggest a start for future research with larger samples but are argued to be transferable to different sets of target groups.