A cover mechanism comprises a support frame with base supports and a multi-part expandable cover engaged with the base supports by grappling members or other means. The expandable cover may have a pliable exterior cover and a set of integral ribs, the expandable cover being foldable when not deployed and being expandable to form a self-supporting structure when deployed. The base supports may provide a sliding track for the grappling members to facilitate expansion of the cover. The base supports may be adapted to rest on the ground or buoyant so that the cover can be used with watercraft. The expandable cover may be deployed by pivoting it from a folded position to an upright position, sliding the cover along the base support tracks, and pivoting the cover from the front of the base supports downwards, forming an enclosure thereby.