Sentimental items
Along the tidying marathon you will find sentimental items. It can be a T-shirt from your university time that you never wear now, a love letter from 20 years ago (before we started to receive those by emails) etc. I love to keep mine in a nice box. It doesn’t mean you have to buy boxes all the time. It might even be a shoe box that sparks joy for you.
I keep my sentimental items in a box on a shelf in my walk-in closet, a place I can see it every time I dress up. It’s not stored in a high shelf where I can’t see it.