Only thing I can think of is, if I'm remembering right, Littlefoot is going somewhere very far away and has no idea what it is. Unlike in other movies like 9, 1, and 4, Littlefoot has no idea where he is going. While using the term 'adventure' may not have been a good one, I can see why he's acting like it's new. I don't think he ever wandered far from the valley before with no game plan. Remember, he and his grandparents had no idea where they were going. They were pretty much "walking blind".
Though Littlefoot should have been more specific, like call it a "mysterious adventure" instead to indicate he didn't know the destination.