In-car exercises are a promising way to keep the inactive driver of future autonomous cars in good shape and alert of the situation around them. To explore how to implement exercises into the car context, we designed AutoGym, an in-car fitness program that translates frustrating traffic into a fun exertion game. To progress in the game, the players must anticipate changes to the traffic situation they are exposed to in the car and work-out against their prediction. In this paper, we present the conceptual design of AutoGym and report what we have learnt from an explorative user study with 28 participants. Furthermore, from the design process and the evaluation, we derived three strategies for implementing exertion games. We found that these strategies helped to conceptualize exertion games as a playful embodiment of the dynamics of driving and as such, can positively influence the experience of control, orientation and situational awareness i.e. experiential factors that can be crucial for facilitating future autonomous driving in a pleasurable and safe way.