Trevor:
I guess the choice depends on whether your family wants to stay in a hotel overnight. Since you will start your trip in Lewistown, you are pretty limited in options for a one day Amtrak out and back ride. With the schedule of the Pennsylvanian, getting back the same day means going no further than Harrisburg to the east or Altoona to the west, and you have already done that. But since you, at 12, will not be a little kid any more, maybe you can talk your family into a night in a hotel.
If you can, I suggest heading to Philadelphia for a day, spend the night at a downtown hotel, and head back the next day. The Franklin Institute is a great science museum with the famed walk-through heart, a huge steam locomotive right inside the building, and an Omnimax theater with pretty neat movies. And, since you are obviously a train fan, you will love 30th Street Station. You also could have dinner at the Hard Rock Cafe which is located in what once was another major Philadelphia train station, Reading Terminal.
Going west is not quite as good for seeing something in the city. Pittsburgh is a great city, but the train does not get you there until evening and, except for Sunday, leaves early morning. Not much time for fun there, but you would still have a great train ride and you will go around Horseshoe Curve. If you stay at the Sheraton Hotel at Station Square you will have freight trains passing right by your hotel all the time. That alone is fun for a train fan! Pittsburgh hotels are also a little cheaper than Philadelphia hotels.
So, those are my two suggestions. Philadelphia and Pittsburgh. And, just a little early, Happy Birthday! One more year and you're a teenager!
Bill
Franklin Institute
Station Square