This paper proposes a service discovery protocol for sensor networks that is specifically tailored for human-centered pervasive environments and scales well to large sensor networks, such as those deployed for medical care in major incidents and hospitals. It uses the high-level concept of computational activities (logical bundles of data and resources) to give sensors in activity-based sensor networks (ABSNs) knowledge about their usage even at the network layer. ABSN redesigns classical service discovery protocols to include a logical structuring of the network for a more applicable discovery scheme. Noting that in practical settings activity-based sensor patches are localized, ABSN designs a fully distributed, hybrid discovery protocol based on extended zone routing protocol (EZRP), proactive in a neighbourhood zone and reactive outside, so that any query among the sensors of one activity is routed through the network with minimum overhead, guided by the bounds of that activity. Compared to EZRP, ABSN lowers the network overhead of the discovery process, while keeping discovery latency close to optimal.