Drifter,
Within McCarran, there are floor to ceiling windows everywhere, so a good view is almost assurred. For the best view, I strongly suggest you make your way to the main terminal and take the elevator to the top floor of the parking garage. Great/fantastic views of the runways, concourses, landings, takeoffs, etc., plus a wonderful view of the Strip as well. There is a viewing/spotting area on the southeast side of the airport along Sunset Road, but you would need a car to get there.
If you are having a 12 hour layover in Las Vegas, you may want to grab a cab or take the Bell shuttle to the MGM Hotel, not too far from the airport. There are nice sights, shopping, restaurants there, are well as the New York New York (mad roller coaster), the Paris Hotel, and the Bellagio. All a very short walk from the MGM.
Hope this helps, if you need/want more info, post a reply or email me. Have a great trip!
Jack