Tell them about the trash room, and where it is if you're familiar with the resort.
If staying in a studio, location of the laundry room and the fact that it's free.
Pack laundry supplies, as they only get enough detergent for one load and don't get any dryer sheets.
Not to pack lots of clothes, as they'll have access to a washer and dryer.
Links to grocery delivery services like WeGoShop or GardenGrocer, or the location of your favorite grocery store.
Not to ask for an upgrade or make other changes regarding the room at check-in, as this could impact your points.
The cancellation/no-show policy. If they're going to be delayed in checking in, they must call the resort (or have you call Member Services). If they don't show up on the first day of the reservation and haven't called, it's cancelled and all points are forfeited.
They can purchase extra housekeeping if they want.
How to use Disney's Magical Express, if they're using it and haven't in the past. To ask for extra luggage tags as soon as the package arrives if they have more than one bag per person to check.
Transportation options at the resort / how to get around WDW.