Exercise video games have a recognized potential for widespread use as tools for effective exercise. Current exergames do not consistently strike a successful balance between the "fun gameplay" and "effective exercise" aspects of the ideal exergame. Our research into the design of better exergames applies existing gameflow research and established exercise guidelines, such as those published by the American College of Sports Medicine, to a collection of four custom exergames: Astrojumper, Washboard, Sweet Harvest and Legerdemain implement full-body motion mechanics that support different types of exercise, and vary in intended duration of play, game complexity, and level of physical challenge. Each game also implements a difficulty adjustment system that detects player performance from in-game data and dynamically adjusts game difficulty, in order to balance between a player's fitness level and the physical challenge presented by the game. We have evaluated the games produced by our design approach through a series of user studies on players' physiological and psychological responses to gameplay, finding that balance between challenge types (cognitive or physical) is an important consideration along with challenge-skill balance, and further, that game mechanics able to support creativity of movement are an effective means of bridging between gameplay and exercise in order to improve the player experience.