Check out this one https://www.wayfair.com/storage-organization/pdp/latitude-run-787-w-x-7086-h-x-787-d-linen-cabinet-w007043887.html

or

https://www.wayfair.com/home-improvement/pdp/clothink-towel-racks-wall-mounted-for-rolled-towels-3-bar-towel-storage-for-bathroom-hinx1249.html

https://www.wayfair.com/storage-organization/pdp/17-stories-agassiz-metal-adjustable-freestanding-over-the-toilet-storage-w100040512.html

When I have guests, I put a set of towels in the guest bath, but then I put an additional set or two in the guest room. Guests have what they need, I can easily check and replenish, the bath isn’t crowded.

I also always stack a set of towels in the guest bedroom as well as in the bathroom. I like the rack ideas posted by Celery. Visualization above, but whether stacked in the bedroom or on a (vertical) rack in the bathroom, those are folded or rolled CLEAN towels. A guest taking a shower also needs a place to hang their damp used towel to dry. In this bathroom, just adding a hook to the back of the door would serve that purpose.

There isn't really a guest bedroom but the owner of the home who is also my mother in law wants guest towels. There is a over the door hook. I'm thinking a white slim shelf. I think that doing something dark would overwhelm the small space. I also want to be able to bring it somewhere else that matches my style of coastal, light wood, bright and airy.

Thank you all for the suggestions!   

How many towels does a guest need? Do you have a drying rack? Assemble it outside the bathroom. Towels can dry on it or stay folded on the top rungs.

Would a train rack over the toilet work for you?

Consider moving the hand towel hook so it's right next to the mirror and visible while standing at the sink (easily reached). Then add 2 shelves on that same wall that can accommodate 2 sets of towels (bath, hand, washcloth). Will keep the space more open vs having another cabinet there.

You need hooks as close as possible to the shower not sure what you mean by no bedroom used for this bathroom. If this is basically a powder room with a shower I use disposible hand towels in a PR. The train rack is what I have in my master bath for extra towesl and you could easily do that on the wall where you have the little wood cabinet. Maybe explain exactly what this space is used for that might help. For guests I store those towels in the guest room .

Over the door.