Why not have a shelf directly over your towel bar? We love this combined Glass Shelf and Towel Bar by Motiv. The combination of functions fits a lot of bathroom storage into this compact design. Now we won't be able to look at our plain old towel bar without thinking about how much more it could do.
We're not sure what's sitting on the shelf in the photo up top, but we'd use it for extra soaps, a bud vase, or a bathroom clock. The shelf depth is 5.7", which isn't huge but every little bit counts when it comes to tiny bathrooms.