These are handy and great for small bathrooms
Think upward. You can build storage above vanity mirror, toilet, even door.
Absolutely agree with Stephenie. Think up. There is so much unused space near the ceiling. Open, running and floating shelves are great. Need to hide something? Use a valance curtain on a tension rod or hung from the ceiling. Just don't put something where it will block an air vent and you'll do just fine.
Don't forget to think vertical inside your vanity. You'd be amazed at what will stack in a small area (bath tissue, tissue boxes, cleansers, bath soaps, Q Tips in the 5" cube shaped container, etc). Placing additional bottles of shampoo and hair conditioner or Band-Aids into compartmentalized see thru bins (some of which have sections that are elevated like bleachers) will also fit under the vanity.
Here are some ideas for bathroom storage I wrote about a while back. https://www.homeroad.net/2011/02/tiny-master-bath.html
I hope this helps!