Socialization is a crucial part of a puppy's development and will give your puppy the best chance at being a confident, well-adjusted adult. A puppy is most willing to try new things before twelve to sixteen weeks of age so it is important to get them out there right away to experience the world!
This checklist shows twelve things (with twelve examples) that your puppy should experience before they are twelve weeks old. Note that every situation must be a positive one! If your puppy is not enjoying the experience never force it or it can be detrimental to their development. Treats and reassurance can help if your puppy is frightened but if that isn't enough you can increase the distance between your puppy and the stimulus or remove them completely.
Keep in mind that the examples listed are only suggestions! Any new and different experiences your puppy can have in a positive way are good for them.