Serious video games are an increasingly popular mechanism for encouraging human behaviour change. Designed well they can satisfy effectively both education and entertainment requirements. However, generating practical ideas for such games is a complex iterative process. There are many methods for generating ideas e.g. mind maps, role-playing, drawing a picture, attribute listing, storying telling, brainstorming. Some of these work best with individuals and some with groups. One popular technique for brainstorming is a card game. This paper proposes an ideas generation card game tool for designers of health behaviour change games. The card game is grounded in explaining how human behaviour change techniques (BCTs) might be used in games. A particular challenge is to determine how many cards are needed for the card game and the scope and level of detail of explanation about each BCT to include on the cards. This paper describes the results of two different experiments to address this challenge and the plans for further experiments.