There will always be special things that don’t fit into any category except, “they’re precious to me.” People with children find this category can become explosively large, so we need to be constantly redefining what actually is special enough to preserve for the ages.
I’ve found it’s useful to have a box for each child. When the box fills, see if you can thin it down a little. When it’s absolutely full, tape it up, label it clearly, store it somewhere safe, and start a new one.
Here’s one box I set up for a client:
And here’s where they live, in each child’s room. They’re easy for an adult to reach and add items to.