In this work we investigate the diffusion of 2D surface islands and voids as a result of atomic diffusion on terraces, by kinetic Monte Carlo simulations based on a hexagonal lattice. We show that the diffusion coefficient of small clusters strongly depends on their size and does not follow a monotonic law. In particular, the diffusion coefficient of small voids increases with the cluster size, reaches a plateau and then decreases. We also study the displacement velocity of clusters when a force biases the probability of the atomic jump direction. We show how the cluster velocity depends on the force and on the cluster size. According to the force direction and amplitude, the shapes of the clusters can be strongly modified.