Give them a staff directory with full names and contact info, along with job titles. Highlight for them people with whom they will be working closely, and in what capacity.
After setting them up with their staff accounts/email, etc., show them if there is any particular style they should use in their email signatures, and if email is going to be a big part of their job, give them examples of communications they will be seeing. This can help with the “vibe” of the company so that they feel like they know how people communicate with each other.
If there are unwritten rules about slack vs email vs phone call vs meetings, give them a sense of that.
Show them organizational systems that people use. Let them know where they can customize things to their liking vs. having to do things a certain way.
Show them how to sign up for benefits and point them to a person who can answer benefits questions for them.
If there are rules about when to take lunch/breaks, let them know. If there aren’t any particular rules, let them know that too. If there is a cultural expectation that everyone goes out/eats lunch together or separately, or if everyone eats in the break room, or at their desk, or what have you, let them know.
If there are cultural expectations about closed/open doors, headphones, kitchen use, impromptu questions, etc., explain them.
Let them know how to call in sick and request vacation days. If they are expected to respond to email outside of work hours, be clear about that (and if they are nonexempt, make sure they know that they must be paid for ALL work, including email).
If they will ever need to use mail or shipping, show them how to do that.
If they deal with customers/clients, give them a script to use with them when they don’t know the answer to something.
Be clear about what equipment/supplies are provided, where it is kept, or how to request replenishment. If they are allowed to purchase supplies that they need/want and be reimbursed, let them know how to do that.
And of course, show them where the restrooms are!