Hmm.. That's actually a really tough question. Only one?
I guess when it comes down to it, it would be watching Wishes.
The trip would feel incomplete without Wishes. We always go to MK on January 30th. They do a pre-NYE "warm up." They go through Wishes and then do the NYE show. For me, that's the grand finale of our trip. Even though we don't leave until the 1st or 2nd of Jan, this is usually our last night at Magic Kingdom. I grab a coffee from Starbucks, watch Wishes, grab a snack from the Confectionary on my way out of the park and take a few steps backward to admire the castle one last time.
We have a lot of rituals - eating at Narcoossee's, seeing Illuminations and Fantasmic, eating around the world, etc. But I'd feel really bad about missing Wishes. The trip just wouldn't feel right.