In this talk I will mainly present a series of experimental results regarding the dynamics of passive particles in liquid bath of active ones. The active particles act here, via the flows they generate, as localized and erratic sources of momentum for the passive beads leading to non-trivial dynamics. Beyond their exciting features for the physicist, active/passive systems are worth studying quantitatively for applications as diverse as the transport of passive entities in cells, biogenic mixing (i.e. mixing of the ocean by living creatures), virus infection, cargo transport (e.g. drug delivery) or self-assembly (e.g. via motility-induced phase separation). The model system I consider is composed of the motile micro-alga Chlamydomonas reinhardtii, a model organism at numerous levels, and polystyrene beads. I will first present results about the diffusive dynamics of micron-size spheres in homogeneous suspensions of algae. I will then show how the coarse-grained dynamics is related to the near-field hydrodynamics of the swimming organism. Finally I will talk about recent results regarding systems of large colloids in spatially heterogeneous suspensions of algae and show how our findings could be (perhaps) put at stake to actually perform useful tasks.