For a first timers first ride ever, I'd go with a classic Disney ride first. Small World, Pirates, Haunted Mansion, Jungle Cruise, or Peter Pan (I know in this case it isn't an option). I'd try to hit a few of those and take time to just "smell the roses" for a first visit. I'd Fast Pass Space Mountain, Splash and one of the above. You should be able to get most of that done in a half day and really get the true essence of "Disney". I wouldn't jump right in with the big rides, depending on their appreciation of nostalgia, they may not "get" the classic Disney rides. Not a popular opinion, but unless you can get a FP, I wouldn't bother with SDMT on a first half day visit.
I would have a very different answer for a first ride of the trip if it wasn't a newbie.
Both my first ride ever and my husbands was Small World.