paper the whole room. dividing it with a simple chair rail will look dated.

unless you paint the rail, lower wall and the base trim all the same color so it appears to be a finished wall.

is the plan to keep the existing paint color? What pattern of wallpaper have you been looking at?

I suggest a chair rail with added moulding below. The choice of wallpaper is endless depending on your style.

It’s really a personal decision based on your patience and skill.

Wallpapering: I did a powder room years ago and it was no fun getting around the toilet and sink, while trying to match up the repeat especially. Paper behind sink with young children can be an issue, unless its waterproof.

Chair rail/paper above: be sure you like the effect as it can feel a bit dated.

The paint color, paper, and rail height all have a big influence.

Painting: if you decide, go bold or moody and add a large piece of art. My two cents, I’d paint and it doesn’t have to go with adjoining rooms.

I wouldn't just do a chair rail.

these are all pedestal sink ideas"

(and I posted a LOT!)

do a wainscot if you like that look.

you have the black floor. do black walls and a white picture frame molding wainscot like here:

maybe a new mirror and some larger art.

reverse colors. white chairail w/black pic frame molding and paint. this one is pretty easy. you just do picture frame squares on the bottom portion of the wall, attach and paint. get a bold light and mirror

you have a black floor, so how about a white board and batten wainscot, some type of black wallpaper, a new light?

ignore the paper, but I love this style of console sink. I'd replace yours and do this to take it up a notch

another console sink w/white wainscot and white/gray paper, gold accents. very classy

If you like your pedestal sink, jazz it up w/a blush pink ceiling, white beadboard wainscot, and some type of print (like this crock or similar?) wallpaper. Bring in gold, maybe a pendant light, shelf over toilet, woven basket, towels, etc.

diff pattern of wainscot/beadboard and dark blue or black walls

beadboard with more of a coastal look:

frame/batten molding wainscot in black with white/gold.

horizontal plank wall treatment. you can do this around the entire room, or just the sink wall

this one is nice and simple. do the white planks, get a new light/mirror, and if you can swing it, a vanity w/marble look sink top. new art, and a rug.

or, these are rather plain wall treatments w/diff paint and accessories

You can pick any bold/moody/dark color you like. I love the black w/a pop of this emerald green and the sconce light. would look rich w/your floor tile.

for the ceiling do a bold paper in the green (or whatever color you choose

OR, paint the ceiling a bold dark color, get a mod light, and try this

Plain old paint jobs and some diff accessories. these are blah, but easy

You can wainscot and then paper above it.

more of a plank board wainscot

Or, just paper the sink wall (you might be able to fit a peel and stick mural)

and then paint the other walls something like this^ Bring in accessories as shown

Wallpaper the whole room like shown here w/a really bold paper choice. I love this chinoiserie look:

You could wallpaper the ceiling in a bold paper, and then paint the walls.

heck, get a diff sink like that marble, and do a satin or gloss bold wainscot. this would look super cool w/your black floor

monochrome wainscot/wall color in peacock:

there's a ton of things you can do, but you have to decide what ballpark you want.

any of these float your boat??

Hmm. Toilet tank is against the wall. You'll have to remove it in order to paint there. There does not appear to be enough clearance between tank lid and wall to permit wainscoting.

What you have is very nice.

Do you have wainscoting or chair rails in the rest of the house ? I do not like that type trim just pluncked in a tiny space . I love all the walls wallpapered in some cases but again it depends on the wallpaper and the space that gray and white one from Lyn would be in scary dept. I get the price of wallpaper but to me it is once and done sodo what you love always even if you have to save a few more months to make it happen.

If you are doing a chair rail and paneling, what goes below the chair rail needs to have a lot of contrast to the wallpaper above. Notice pics below. The most punchy pics have a lot of color contrast between the two.

Another OP missing in action . Beth I know you mean well but honestly all those ideas are mind boggling and make it even more confusing usually . 3 pics of 3 very different ideas would be great. But we need info fron any OP to really help.

That matter of no clearance behind toilet tank seems like a tough one. Can't even use a paintbrush without removing the tank. How would y'all handle?