I need to see the whole space posted here in comments DO NOT start another post .

Open shelving or frame it and put a door on it for closed storage. It's easy to lose things in the back of 24" deep shelves, so if you think that might happen, put a false back in it to make the space shallower. Pull-out shelves wouldn't work in this hard to access space.

Additional photos

What do you still need in guest bath but no where to store it? Suggest avoiding dust collectors. Suggest for hygienic purposes (not everyone closes toilet lid before flushing) nothing you can’t disinfect with clorox

Leave the base shelf empty for the guest to put their toiletry kit there. A framed travel print could go in the niche.

Wow, it’s so tempting to see it as useful storage but I think it will be hard to put much there AND have it look good unless able ti do custom closed cabinetry. And even then.

I like apple_pie’s keep it simple. I’d probably try to find a waterproof tray that fits the decor & looks good, since ggd area looks like drywall or something. Or don’t even need a rim— a piece of granite or quartz, but finished. Just brainstorming, so not sure what. Might be something in Kitchen ware— like a marble/ faux marble piece or even tempered glass. To set guest’s things on as noted, since there’s not much counter space. Could have a small decor item on it that can be moved easily if someone’s staying with you. Not a faux plant though. Then the art above. The point is to have it look good and intentional, but embrace the openness and not gew-gaw it up. It is what it is.

Roll up some nice fluffy towels and stack them in that area. Get nice towels to match the bathroom if it is a guest bathroom.

What's on the other side of that wall ? If it's a bedroom I'd close it up.get the sqft om the other side where it's accessible

I like the idea above of purposeful shelves, with maybe a false back to shorten the depth, except maybe at the bottom for the rolled towel idea from Jackie. Something decorative possibly and a picture in the toilet area. Maybe bright white paint to help with lack of window...just have fun with it!

I avoided the towel stack idea thinking the towels might not be used often— specifically big bath towels. So I don’t like open towel storage that looks so nice in photo shoots because I don’t think they stay clean for several months.

Now, if you intentionally rotate your own bath towels between 2 sets, that might work. Or, if you found a covered large square chest that just fits in the cubby, or a lacquered box- and could be dusted ( I don’t like most baskets out permanently in bathrooms because get dusty & can’t be cleaned easily), then you could store most anything. This is the kind of dilemma where you might have several possible ideas, maybe one is ideal but you aren’t able to execute it right now, but it’s in mind in case the parts turn up.

Another decor approach might be immediately doable without being too permanent ( hanging a fun but inexpensive art print).

Guests can always use more space to stash their bathroom items when staying at your home. I'd put shelves in and believe me, guests will fill them up with their stuff. I would make a false back so they are more shallow.