While problem solving is a crucial aspect of programming, few learning opportunities in computer science focus on teaching problem-solving skills like planning. In this paper, we present Pyrus, a collaborative game designed to encourage novices to plan in advance while programming. Through Pyrus, we explore a new approach to designing educational games we call behavior-centered game design, in which designers first identify behaviors that learners should practice to reach desired learning goals and then select game mechanics that incentivize those behaviors. Pyrus leverages game mechanics like a failure condition, distributed resources, and enforced turn-taking to encourage players to plan and collaborate. In a within-subjects user study, we found that pairs of novices spent more time planning and collaborated more equally when solving problems in Pyrus than in pair programming. These findings show that game mechanics can be used to promote desirable learning behaviors like planning in advance, and suggest that our behavior-centered approach to educational game design warrants further study.