To -do list before leaving Belgium

all insurances,car, house, family, hospital. Mutuelle. television. Telephone fix line. Mobile phone.Internet contract. Schools. Post office. Gas and Electricity. Three month notice to cancel your house renting contract and garage if you are renting one. Banks.Post office with new adress .Get medical report from your GP and dentist(xrays) and hospital exams if you had any (over here, you are allowed to have copy of all your medical history ) . Job references or contract here in Be. veterinary vaccination book from cats and dogs. friends adresses. (ask your post office I thinck they have a sort of file with most things you need to do when you move houses.
I don't know about your commune but I would go and cancel your adress and ask about id card