Detailed cleaning instructions. How often and what you do is good. Something like take rabbit out, remove litter box and scoop out, put fresh litter (handful) in box, sweep out cage and wipe down if needed, put litter box back and add hay, scrub out food and water dishes, put fresh water in bowl. Basically whatever you do, but make it as detailed as you need to.
Have the food, hay litter and whatever else is needed near the cages. This allows for it all to be avaliable right there. Even if you do not normally keep it there, do it for the time you are gone.
You may want to have the veggies dividend into individaul meals. Then the sitter just needs to wash them and give them to the bunnies and each bunny gets what they need.
Since it is only a few days, you don't need to go overboard. Do a good cleaning before you go so the sitter doesn't have to and only needs to spot clean. You should not have to worry about having enough food and litter, but make sure there is enough for at least a few days after you come back.
Include vet info. As it is over the holidays, have a rabbit savvy emergency clinics number as well just in case. Leave your credit card number with the regular vet and maybe some cash if needed for an emergency. The phone number, address and good directions are a must as some vets are hard to get to. Have the carriers out so the sitter doesn't have to find them.
You will probably be more of a wreak about this than the rabbits. They will probably be fine, but it doesn't hurt to have all your bases covered. It is better to be over prepared and not need the stuff than have to scramble becasue you did not think of it.