I would not let them take him outside. You never know if a pen is latched properly, what other animals may be around or how competent the sitter is with rabbits. A rabbit can slip out of a harness, so that isn't a great option as well. It is one thing for you to do these with your rabbit, but it can be a liability issue for someone else.
Try setting up a pen inside that can connect to his cage. This way the pet sitter only needs to open the cage to let him out. They would not have to pick him up either, so it is safer for both of them. It would be good to set up the pen now so he can get used to it before you leave. It would be ideal to have the sitter come twice a day, they can let him out when the come and put him back later on.
To make it easier, divine up veggies into bags. Then they can just take out a bag and feed that rather than having to figure out how much and what to give. You can wash the veggies, but wrap them in paper towel. Otherwise just put some paper towel in the bag and the pet sitter can wash them.
Make sure to have enough pellets and hay to last plus extra. Have them in containers that are near the rabbit cage and label them if you need to.
Since you will be gone for more than 1 week, cleaning will come up. Give instructions on who to clean and what should be cleaned. It can be to clean out the food and water dishes, clean the litter box (or at least scoop), give fresh bedding or whatever else is needed. You should make sure the cage is clean before you leave and be prepared to clean it well when you get back.
If you can, have the pet sitter come over before you leave. Go over what care is needed, where things are and allow your pets to meet them.