How can I decorate a very small 1/2 bathroom?

a cute mirror and light fixture can go a long way. Maybe even making one wall an accent wall with wallpaper or shiplap may help.

Add a few shelves if you have space. Decal on the wall wouldn't take any space but add decor touch.

Even if the cabinet has a mirror, hang another big mirror—over the sink, on a wall. Hang a pocketed or shelved storage unit on the back of the door. Keep the floor light unless you want to be dramatic. To be dramatic, paint the walls and floor black, navy, or your favorite color, or use foiled wallpaper on the walls. Use a very small trashcan, a square box of tissues, a basket or shelf for toilet paper and wipes. I just saw a project this morning on Hometalk about using baskets in a collection on the wall that might work for you. Jewell

I would keep your paint colours as light as possible to make the room look larger