Plenty of things like washing up liquid, dishwasher tablets, hand soap, toilet paper, some cleaning material, bin bags etc.
If you have a washing machine - some laundry pods
More than one towel per person
Plenty of crockery, cutlery, glasses and mugs. It doesn't have to match but I don't want to wash up immediately.
If you water for family - some children plates/mugs
Some food containers
Hooks in the bathroom for wash bags and plenty of space to hang up a towel to dry properly.
A decent size wardrobe so you can unpack properly
Some nice throws/blankets to use on beds/sofas
Offer highchair/cots (not a travel one, a proper one), some toys
I wouldn't automatically put milk in the fridge, I would offer to provide it on request. Tea/coffee bags yes, a closed pack of biscuits but nothing which can spoil.