You can hand in your keys at the caretakersoffice. When you ended your tenancy agreement online, you received an e-mail of De Key with this information. Can't find this e-mail anymore? You can always contact your caretaker or Woonservice for this information.
If the caretaker is not at the office, you can also hand in your keys by putting the keys in an envelope (with your address on it) in the caretakersmailbox.