For little ones, my grandkids always liked the Lionel operating playground with swings, seesaws, etc. I also picked up an inexpensive recordable sound card and recorded children's playground sounds which goes off whenever someone operates the playground, itself and added some bikes and a ice cream truck.
They also quite liked the gateman (mine is MTH); the movement of the Lionel Icing Station; and the Lionel Operating Platform where the passengers seemingly disappear by magic by pressing a button after the train departs. Saying "Where did the people go ?" always got their inquisitive minds going.