Consider the following playbook which we can execute without an inventory:

---
- hosts: localhost
  tasks:
    - name: get value
      debug:
        msg: "The value is: secret-value"

To execute this, we can just run the ansible-playbook command with this playbook:

$ ansible-playbook playbook.yml
[WARNING]: Host file not found: /etc/ansible/hosts
[WARNING]: provided hosts list is empty, only localhost is available
PLAY [localhost] ***************************************************************
TASK [setup] *******************************************************************
ok: [localhost]
TASK [include secret] **********************************************************
ok: [localhost]
TASK [get value] ***************************************************************
ok: [localhost] => {
"msg": "The value is: secret-value"
}
PLAY RECAP *********************************************************************
localhost                  : ok=3    changed=0    unreachable=0    failed=0