Excitement. Overexcitement, rather. The kind of overexcitement shown in the situation "OH MY GOD THIS THING JUST MADE MY TITS BIGGER! I'M GOING TO HASTILY USE IT AGAIN WITHOUT THINKING THINGS THROUGH!"
Honestly I've never given it much thought probably surprise? My least fave would be oblivious because it would be hard to drive the scene with a sense of apathy or not reacting to the changes in a story.