I recently got a new house and (unlike my current house) the kitchen has cabinets going all the way to the ceiling. These cabinets are unreachable without a ladder and each one has windows so you can see inside and each has a little light to illuminate the interior, so the intention appears to be that you would put a kitchen item in there to be seen, but not often used.
What would go in a place like that?
The intention, I think, is that you'll use them for your most ornamental presentation dishes, that are saved for best.
If you don't have such things, I'd suggest using them for the rarely used items rather than for pretty things, but that's because I have more rarely used gadgets than decorative kitchen-/table-ware, and am of a practical nature.
In practice I also use my more inaccessible kitchen cupboards for reserve ingredients - the spare packs that I'll need next time I use a large quantity, the cupcake sprinkles, anything that I'm not likely to want suddenly.
But it's very much a matter of personal taste - what do you like about your kitchen? Its appearance? Its place as the centre of a home, where delicious things are made? It also depends how much you need the space - in an oversize kitchen you can use them for purely decorative items, whether that's vintage china or jars of dried chillies you selected for show.
After doing some more research on this question, I have found that this is actually a pretty common problem because a lot of people build to the ceiling, otherwise the cabinets will collect dust on top. It turns out that the display cabinets up there are mostly used for the following:
