1. An activity monitoring system comprising: a plurality of fixtures, wherein each fixture includes a physical size and shape that is adapted to couple to an associated location on the body of the user; a portable monitoring device adapted to separately and physically engage each fixture of the plurality of fixtures, the portable monitoring device including: a housing to separately and physically engage each fixture; an activity sensor, disposed in the housing, to detect activity of the user and, in response thereto, to generate data which is representative of the activity of the user; and processing circuitry, disposed in the housing and coupled to the activity sensor, to calculate one or more activity-related quantities of the user using the data which is representative of the activity of the user, wherein the processing circuitry: automatically determines a particular fixture of the plurality of fixtures is physically engaging the portable monitoring device, and calculates at least one activity-related quantity of the user which is associated with the particular fixture.