Since I live in earthquake country, I keep emergency supplies up there. Canned goods that get rotated every 6 months, including pet food. At one time it held liquor bottles. In another house I had odd serving dishes that were only used for large group entertaining. Vases and other vessels not used on a daily basis. Always with child proof locks so the contents didn't come spilling out in the next quake.
I keep all my kitchen ideas that are not used very often but are to good or expensive to just through away.
Cereal boxes and less used baking items and equipment.
Cereal boxes can be "retrieved" using long silicone tipped kitchen tongs.
I would always forget what I had up there so when I did a kitchen update I decided to take out the shelf, take off the doors, paint it inside, and decorate it!! I've attached a picture.
I always keep things like crystal and other fragile things up there that are not used often. I leave a couple of the glasses in a more accessible cupboard for use with wine or aperitif when the occasion arises so that I don't have to go into that top cupboard.
Keeping a folding 2 or 3 step stool nearby makes accessing items in that space much easier.
I have my Christmas coffee cups and other holiday dishes.
Seasonal. Canning supplies. Stuff you only need very rarely because it requires a stool.