We consider wireless sensor network applications where mobile robots serving as sinks are responsible to collect readings from sensors in a given area and delivering the readings to a central authority within some bounded delay. We refer to these mobile robots as collectors. This scenario raises a number of challenging issues, as we need to define specific coverage strategies to guarantee that: (i) collectors visit any point of the target area on a regular basis, (ii) collectors to cover the entire area, and (iii) trajectories should take into account the fact that collected data might have spatial correlation. We propose to define trajectories for mobile collectors based on the concept of space-filling curves to address the problem of zone coverage. We investigate the appropriateness of different space-filling curves to respect a number of requirements (at both the system and applicative levels). By numerically analyzing a number of scenarios, we show that our proposal is both effective in guaranteeing full coverage and in respecting applicative requirements. Our results assess both the delivery delay and the aggregation accuracy of the investigated curves.