Ubiquitous smart environments, equipped with low-cost and easy-deployable Wireless Sensor Networks (WSNs) and with widespread Mobile Ad-hoc NETworks (MANETs), are opening brand new opportunities in urban monitoring. Urban data collection, i.e., the harvesting of monitoring data sensed by a large number of collaborating sensors in a wide-scale city, is still a challenging task due to typical WSN limitations (limited bandwidth and energy, long delivery time, …). In particular, effective data collection is crucial for classes of services that require a timely delivery of urgent data, such as environmental monitoring, homeland security, and city surveillance. This paper proposes an original solution to integrate and to opportunistically exploit MANET overlays that are impromptu and collaboratively formed over WSNs in order to boost data collection: overlays are used to dynamically differentiate and fasten the delivery of urgent sensed data over low-latency MANET paths. The reported experimental results show the feasibility and effectiveness (e.g., limited coordination overhead) of our solution for MANET overlays over WSNs. In addition, our proposal can easily integrate with the latest emergent WSN data collection standards/specifications, thus allowing immediate deployability over existing smart city environments.