New Senses to explore
Ensure your puppy is exposed to as many different sounds, sights, and materials. Have the puppy walk on grass, sand, rocks, mud, etc.. Expose dogs to balloons, backpacks, and large tables. Have the puppy hear different kinds of music, loud noises like thunder or fireworks. Make sure these are positive experiences.
New People
Have your puppy meet all kinds of people. Elderly, teens, male, females, young children, tall, short, etc.. The more diverse kinds of people your puppy meets, the less they will be afraid of different people when they are older.
New places
Bring the puppy to friends and families homes. Ensuring dogs are safe (look below for more information on that). Bring your puppy on as many car rides as possible.
Known Vaccinated Dogs
Have dogs interact in safe places (at private homes or land) with different kinds of dogs that you know are fully vaccinated and healthy. This is a great way to have your dog interact with new and different dogs in a safe way.
Important Considerations
Carefully watch your puppy when introducing new things to them. If they are over stressed they could make a bad association and become fearful in the future. You want to make socialization a pleasant experience. Mild stress is ok in order for the puppy to bounce back and learn that if they keep trying then things will be ok.