Socialisation is also way more than just visual.
Think of surfaces they need to walk on - this can be done at home, so get wooden planks, thick carpet, laminate, a trampoline (obviously wit you holding on!) surfaces that move .
Smell is very important, barbecues, petrol stations, smoke, animal smells.
Sound play sounds on phones squaking birds, gunshots, fireworks etc
Put novel items in the house and garden, wheelbarrow upside down, ironing boards on their side, plastic bags (supervised) blowing in the wind in the garden.
REMEMBER all of this should be under threshold and watch out for your puppies reactions, any lip licking, yawning, turning head away, ear position, whale eyes, panting etc increase you distance.