Stock ,Veg, herbs and spices, go easy on the root veg like parsnips etc as they are quite high in carbs. Bovril or Oxo is a lovely quick drink that's handy on pre op diet, they also do chicken and curry flavours too.
I was allowed muller lights on pre op but each hospital varies, if in doubt give the nurse or dietitian a call. Dont be tempted by fruit corners though lol. Good luck.