If you have a mirror over the sink with no cabinet behind it, there’s your solution right there. Replace the mirror with a mirror cabinet and you have stack of storage. Embarrassing items should not be kept here as guests may well sneak a peak as they’re washing their hands! The IKEA STORJORM might do the trick for you and has built in LED lighting.
Another place for a shelf is under the bathroom mirror, above the washbasin, so that your most used and least embarrassing items can be put there in easy reach.
A glass shelf with give the most class. Sure it might give away that you use a sensitive deodorant or Aldi soap, but if they look nice, what does it it matter?
As long as you leave enough space to remove the cistern cover if you have a plumbing emergency, you can put, one, two or three shelves to hold baskets of toiletries, handtowels, face washers, plants, whatever works for you.