Unless you need the storage from shelves, I think artwork would be perfect there!

A clearly written message nicely framed : "Aim carefully." =)

Cabinet with shelves (and solid doors to protect towels from moisture) all the way to the ceiling.

Leave room to remove the toilet tank lid.

The bottom shelf should not extend farther than the toilet tank lid so people don't hit their heads as they're standing up. Higher shelves can be deeper.

One white floating shelf. Some inspo pics:

You said it in your statement.....We have a very small bathroom...which will equal to the need of storage. It does not have to mean you clutter everything but add some. The bathroom after all is to be used...Mon's ideas are good. Art work will make it look nice but will not resolve the storage issues especially since you are intending to do a floating vanity thereby sacrificing storage there as well.

Function first, form second. If you need storage, a wall cabinet with doors to hide the clutter. If you don't need storage, it's a toss up between shelving and a piece of art work. The art has to be able to withstand moisture. If it were me, I'd opt for the cabinet. But, I'm an organization type of gal.

Pretty bathroom! I’d do a glass shelf, to keep it looking uncluttered and airy. If you don’t need a shelf, I agree about a narrow picture or other art, something elegant and simple.