Do you want them for storage, or for knick-knacks? For storage I'd get pretty baskets that hide what's inside. Towels look pretty if you roll them.

It looks like a nice hamper might fit under the shelves. I would have towels rolled on one shelve and baskets with bath essentials on the others.

Thank you

Do you need the space for storage? I think roarah's suggestion of a hamper is a great one. It will add some texture and be useful. (I wish I had room for a hamper in my bathroom.) I would then use the space to store nicely folded (or rolled) fluffy extra towels. You could also put a basket on one of the shelves in a material coordinating with the hamper. Hope that helps.

We have this situation in our recent remodel bc there was a weird space in the wall (AC return, other stuff we didn't want to re-route) that forced us to have some wasted space behind the vanity. We could have just closed it up with drywall, but instead we decided to put some shelves in there. We still have some empty shelves, but this is what we have done so far: candles and bath products. For the remaining shelves, probably we will put a framed photograph, a nice piece of coral or large shell, or some pretty pottery. Below we don't have an open space like you do; it is a cabinet door. If you don't already have a good spot for a hamper, that might be a good place for one. If you already have a place for a hamper, you could put a smaller "dry-clean" hamper there for things you want to separate to take to the dry cleaners.

Ssdarb, how wide are your shelves and how deep. I like the idea of using all space available!

Shelves 12 in deep 22 in wide. The open space on bottom is 36 in high

The shelves are about 8 inches because that's just the amount of space that was available after all the frames, etc was done. The cabinet below comes up to the same level as the vanity cabinet. The reason they are stained instead of painted is because there is a stained cabinet elsewhere in the room, and there are wood floors, and we wanted at least one other place to repeat that warmer treatment in an otherwise white and grey bathroom.