In this paper, we present a framework for planning an activity to be executed with the support of a robotic navigation assistant. The two main components are the Activity and the Motion Planner. The Activity Planner composes a sequence of abstract activities, chosen from a given set, to synthesise a plan. Each activity is associated with a point of interest in the environment and with probabilistic parameters that depend on the plan, which are characterised by simulations in realistic scenarios. The low-level action to pass from an activity to the next is handled by the Motion Planner, which secures the physical feasibility of the chosen actions and their compatibility with the constraints posed by the user and the environment. Indeed, the final plan must respect the user constraints and optimise his/her satisfaction from the activity. We show a possible model for the problem as a chance constrained optimisation along with an efficient technique to find high quality solutions.