This short paper presents some preliminary results (both quantitative and qualitative) gathered from field trials with three different mobile apps that allow walkers to map urban accessibility barriers/facilities, while wandering around. The three apps were designed based on different gamification mechanisms, respectively exploiting: i) intrinsic (i.e., altruistic) motivations, ii) extrinsic motivations expressed in terms of a concrete reward, and iii) extrinsic motivations expressed in terms of fun/entertainment. These preliminary results reveal that the apps designed on the basis of extrinsic motivations are able to drive users to provide a larger amount of contributions. Interesting differences between concrete rewards and fun used as effective means to motivate contributors are discussed.