I kid you not, my earliest memory of the park is trying to get across MSEP in Fantasyland when I was about 4 or 5 years old. My older brother and I were with my DL-local aunt and cousin (though surely my parents and grandma were somewhere in the park too), trying to get from Fantasyland into Tomorrowland, just before the American flag finale float came up past the Matterhorn on the right-hand side of us (so it must have been the 2nd parade of the night). Just as the crossing point was about to open up, my aunt threw me on her shoulders and my cousin grabbed my brother by the hand, and we all scampered to the other side together.
It’s funny what you remember after all those years. It’s not a character interaction or a thrilling E-ticket, it’s the entirely random moments that fill in the rest of your time in the park. That’s the real Disney magic!