SO many...
first I remember that has really stuck with me was a day-camp kids tour thing where we were on a pontoon boat in the seven seas lagoon, followed a pirate map to buried treasure that ended up being a chest full of marshmallows that we roasted on a campfire on the beach, epic (probably circa 1980).
next was a dinner at the Empress lily room, there was a steward assigned just to keep our water glasses full and we were trying to race him to finish a glass before he refilled it and got brain freeze.
I did a backstage tour as a teenager that was focused on art in the parks and had an animator teach us to draw a Mickey (can still do it following the same tips 30 yrs later).
We've done the fireworks cruises and fishing excursions and loved those.
My brother got married in Disney and there were a whole host of unique things most people wouldn't encounter (reception in the living seas)
But my absolute favorite #1 pick would be the Wild Africa Trek. Its not a backstage tour, the entire thing is "on stage" the Cms stay in character like its a real safari, but the time and depth you get to spend on it are amazing, and we loved the adventurous parts like the rickety rope bridge over the crocodiles, and the meal in the boma out on the savannah as we watched the wildlife (a baby rhino came out from some bushes right in front of my daughter and I after the rest of the group had walked away and it was very special and precious to us).