Inside your non-working fireplace.
Because a fireplace is often the focal point of a room, this is a primo spot to display an interesting vignette or showcase something you're really proud of.
On top of the toilet.
This is a great spot for trays, tchotchkes, and even vases with fresh flowers. Flowers in the bathroom is the ultimate luxury.
Behind the door.
Hang lightweight art here, because why shouldn't you see something nice when you're coming or going?
On top of the cabinets.
This is a real zero, dust-collector space, and if you don't need it for extra storage it can be a fun spot to display collections, like plates or globes or books or really anything you an dream of.
Above the fridge.
Nobody says your kitchen has to be a completely utilitarian space. If you're not using the top of your refrigerator for cereal boxes or liquor bottles (a few uses I've found for that space over the years) it can also be a great spot for some plants or treasured objects d'art. (Bonus points if it's a Smeg.)