Travelator at the airport.
Especially when, as happened this morning, there is no one else on it because I love walking as fast I can and feeling like some kind of superhero striding out.
Conversely, I am most disappointed when there are lots of people and my childish glee has to be contained - and when someone has the temerity to block both lanes so I can't get past I feel deflated.
What childish things do you still get excited by as an adult?