Set up a bunch of triggers in the world (i.e. make a bunch of box colliders or whatever and check this "is trigger" sketch on them).

Put something similar to this on those game objects (untested and I don't use javascript so it probably doesn't even compile):

var switchToCamera : Camera;

function Awake()
{
    switchToCamera.enabled = false;
}

function OnTriggerEnter( var collision :  Collision )
{
    if( collision.gameObject.tag == "Player" )
    {
         switchToCamera.enabled = true;
    }
}

function OnTriggerExit( var collision : Collision )
{
    if( collision.gameObject.tag == "Player" )
    {
         switchToCamera.enabled = false;
    }
}

Basically you need to set up your world where you have a bunch of disabled cameras, and when you enter a zone it turns on the camera for that zone.

There's a little bit more to it than that, like maybe having a default fallback camera that's above the player or something. Or turning off the main camera when you enter a zone (probably just by setting Camera.main.enabled = false; and re-enabled on leave).