Great question! I like the ornament idea, but have not kept up with it. Here's a few... just one more week!
We try to take a family pic by Roy and Minnie on Main Street, and by Walt near the castle.
Other traditions:
Must have my Dole Pineapple Whip at Aloha Isle!
Must have my Guiness at Rose n Crown.
Daughter is 10 and has been going since 4. She tightropes just about every curb along Main Street or anywhere else in WDW. I don't know how it started, but it is a tradition now!
Home movies! I videotape my kids on the rides as they become Walter Cronkite, narrating the entire event. At one point on the "Blue Cars" (TTA), my daughter always says, "Hang on, here we gooooOOO!". On top of Thunder Mountain, my son always yells, Mommmmmieeeeeee!
My son must have a Kiluhea Tort for desert at Kona Cafe. Daughter has the Kiddie Cone.
On that sad moment at the end of the trip, I interview the entire family about the highlights of the trip. Rides, food, funny things... Makes a great ending to the video.