Socialize with what you have at home. Make shift obstacle courses, tubes to climb thru, child play sets, chainlink laid down, walking on a tarp, dropping loud things, climbing and going under and through things.
Get creative! Those will really help your pup out before they can see the world.
Even then, if your pup has two sets of shots go out there! Home depot, lowes, and bass pro shop allow dogs and don't get a lot of dog traffic. Perfect socializing opportunity without risking infections.
Get your pup to meet lots of friendly people and make positive experiences. This will help them in the long run to have a better understanding of non threatoning, neutral, friendly, threatoning, dangerous.
The German shepherd temperament of aloof will come at maturity so your dog won't act like a golden retriever as an adult.
"For the animal shall not be measured by man. In a world older and more complete than ours they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear."