Because I'm going to stay in one tomorrow night that is apparently haunted by a poltergeist (I'm sceptical, my mum asked me to go...)
I have to take a sleeping bag if I want to sleep (there are beds but no bedding)
A torch and spare batteries (because batteries drain quickly inside the house apparently)
Food for breakfast, snacks, teabags, milk etc
Phone charger
Pyjamas/clothes and toothbrush obviously.
What else?