You can't go wrong so long as you're close to Shibuya station. That round-trip promo is good as it includes all connecting rides on local JR trains until you exit the turnstiles. The next day, you could use this benefit to board a Yamanote line train at Harajuku after lunch, then transfer to the NEX airport train at either Shibuya or Tokyo Station.

For a snack upon arrival at Shibuya, walk up the pedestrian street from the famous crossing. There are many quick options along the way like Genki Sushi or Ichiran ramen. Look for the Pablo cheese cakes. Or you can just get tasty rice items at any Lawson or Family Mart before checking in at your hotel.

For stuff around Tokyo Station, get to the main concourse then head one level down to the "First Avenue" basement complex. Ikaruga has some of the best ramen, there are many others.