Take the tram ... to the Fashion Show Mall or Starbucks
The free tram from the Mirage to Treasure Island let's you out at the north side of TI. You can quickly navigate past the buffet (hug the left wall) to an exit in the building and take a single skywalk to the Fashion Show Mall. Also extremely handy for TI's Mystere and TI's Starbucks (both just down the escalator), if you don't like Mirage's Roasted Bean.