Charlie, John is right for a medicine cabinet but let's look at another way. If you have a cabinet under the sink, put a mirror on the wall over it. Then you can put some items in the vanity cabinet. Toothbrush holders and glass holders are available in wood, ceramic and probably other materials. You can also get metal or plastic decorative shelving for this at places like Pier 1. She will soon use up all of this space and you will still be fighting for a place to leave your shaving cream and razor. But maybe you will be able to go a week or 2 before the battle starts up again. LOL I am lucky not to really have this problem with my wife, but the teenagers made up for it.
One neat deal that our daughter has is a wicker basket sitting on her toilet tank. This holds 2 rolls of paper that are not visible. They will not replace these from the closet supply when they start on a new roll so watch the trashcan for doot-a-doots (empty cardboard rolls).