This paper examines the affordances of a component in a serious game, specifically how one material design choice affected the interactions and opportunities for agency and learning. The game examined is a megagame, a large-scale (20-100 participants) social learning environment combining board-gaming with role-playing. The megagame poses participants the challenge of creating a sustainable society, and focuses on developing participants' understanding of how different stakeholders in a regional energy system and society are interconnected. Negotiation over conflicting goals was a primary activity in the game, and agreements were formalized through paper contracts. Contracts were designed to act as boundary objects between player teams, and defined their financial exchanges. This exploratory study finds evidence in the interactions between participants that the paper contract system facilitated opportunities for participants to develop understanding about the interdependencies between teams and resources, and to exert agency over their role in these relations. Players actively maintained and prioritized the correspondence between copies of contracts as a means of regulating both the game's economic system in the game and their mutual intersubjectivity. Overall, the study highlights the importance enabling participants to experience how joint actions cumulatively produce future consequences, and how opportunities for agency and negotiation educate about the ongoing global polycrises of energy, climate and social tension.