That's a fantastic list of socialization activities! Here are a few more that I do in addition to yours:

People who are:
walking
running
bending
hopping
crawling
swimming
bathing
carrying things


People who are riding:
bicycles
skateboards
wagons

Surfaces:
grass
concrete
gravel
asphalt
slippery floor
stairs
sand or dirt
rocks
snow
puddles

Sounds:
doorbell
TV
radio
pots and pans
vacuum
stereo
clapping
dropping things
whistle
sirens
singing
car honking
balloons popping
noisemakers (party)
popcorn popping
electric saw
blend