Thick blankets and rugs are great for absorbing sound. Drape them over the door or hang them on a curtain rod above the door.
If there are any gaps between the door and the door frame, or between the door and the floor, you can use towels or foam to fill them in.
Soundproofing curtains are specifically designed to block sound. They are typically made from heavy-duty fabric and have a special backing that helps to absorb sound waves.
A door sweep is a strip of material that attaches to the bottom of the door and seals the gap between the door and the floor.
The more mass a door has the better it will be at blocking sound. You can add mass to a door by hanging a heavy mirror on it or by attaching a piece of plywood to the inside of the door.