Way back when ticketing was different, they used to stamp your hand when you left the park, then you just showed your hand stamp if you wanted to go to another park. Our tradition was on our last day, we would go to a park and get our hand stamped as we left (sometimes flight times only allowed us to spend about 1/2 hour in the park). That way, if our flight got cancelled, we could head back to a park. That never happened, but at least we always had a great souvenir to look at the next day at home.
Unfortunately, since the hand stamp stopped we don't have any particular "must do", just a ride we must ride, deopending on the park. Like in the Magic Kingdom, we must do the Jungle Cruise because when I was little I had a Viewmaster with the JC ride on itt.