I know adding a probiotic powder like benebac, or adding electrolytes to the water can sometimes encourage a rabbit to drink more. Some people mix in a bit of apple juice, but I wouldn't do very much as you don't want to be giving your bun the extra sugars, especially while on antibiotics. If you do any of these though, the water needs to be changed out regularly to prevent bacterial growth occurring. Benebac would be my choice, as it will also help with the bun being on antibiotics. But this should be given a couple of hours after giving the antibiotic, to have the best effect.
There is always the option of directly syringing water into your buns mouth. This has to be done carefully though, so none of it is aspirated. Or you could have the vet show you how to do sub q fluids and get a prescription from the vet for them, so you can get them and do it at home.
But really, as long as your bun is eating and drinking a sufficient amount, the antibiotic should take care of the infection and get your bun back to feeling better. But if she isn't eating and drinking sufficient amounts, then supplementation is needed. If she isn't eating well, she should be getting syringe fed a recovery food(vet should have given you), and she can be given extra fluids this way.