Check out Unity Steer:

http://www.arges-systems.com/articles/35/unitysteer-steering-components-for-unity/

http://www.google.com/search?hl=en&client=opera&hs=5jX&pwst=1&rls=en&channel=suggest&sa=X&ei=Pd6dTYbhLqGE0QGJ0rG1BA&ved=0CBQQvwUoAQ&q=unitysteer&spell=1

Will help =).

I wrote a script that moves objects randomly in a restricted area. In this case on a plane xz becouse I'm working on a "ship's game", but with a few modifications it'll be usefull. The only bad thing is that the object don't avoid obstacles.

using UnityEngine;
using System.Collections;

public class BotControl : MonoBehaviour {

    public float speed;
    public float area;
    private Vector2 newWayPoint;
    private Vector3 wayPoint;
    private Vector3 oldWayPoint;
    public float timeSmooth;
    private float time;
    private CharacterController controller;
    // Use this for initialization
    void Start () {
       newWayPoint = Random.insideUnitCircle * area;
       wayPoint = new Vector3(newWayPoint.x, transform.position.y, newWayPoint.y);
       controller = GetComponent<CharacterController>();
       oldWayPoint = wayPoint;
    }

    // Update is called once per frame
    void Update () {


       SailRandomly();



    }

    void SailRandomly(){
       //rende lookat() più gradevole alla vista
       Vector3 smoothLookAt = Vector3.Slerp(oldWayPoint, wayPoint, time/timeSmooth);
       time += Time.deltaTime;
       smoothLookAt.y = wayPoint.y;

       //il bot si dirige verso "waypoint" finché non lo raggiunge, poi cambia direzione
       if(Vector3.Distance(transform.position, wayPoint)>20.0f && time/timeSmooth < 1.0f){
         transform.LookAt(smoothLookAt);
         controller.SimpleMove(transform.forward * speed);
       }
       else {
         newWayPoint = Random.insideUnitCircle * area;
         oldWayPoint = wayPoint;
         wayPoint = new Vector3(newWayPoint.x, wayPoint.y, newWayPoint.y);
         transform.LookAt(smoothLookAt);
         controller.SimpleMove(transform.forward * speed);
         time = 0;
       }
    }

    void OnGUI() {

       //GUI.Label(new Rect(Screen.width-400, 0, 400, 200), "" + oldWayPoint.ToString() + " | " + wayPoint.ToString() + " | " + time/timeSmooth);
    }
}