I didn't think much of it at the time, but when the kids were young, the Sci-Fi Dine In Restaurant served kids meals in glow in the dark frisbees with the restaurant logo on it. I have held onto those, even as the kids' interest faded years ago (the glow in the dark has NOT faded!)
I also still have that glow in the dark frisbee from the Sci-Fi Dine-In Restaurant.
I also have a poster from the 25th Anniversary Celebration, a 2000 Celebrate the Future license plate and knick-knack that has Mickey hands holding the Earth. I still have park maps from various years from Walt Disney World and Disneyland. I have kept park tickets and Fastpasses, pins and even Block Party Bash balls. I even have my first Goofy hat from when I was a kid.
It's funny what we keep and what stays valuable to us because of the memories they evoke.