hahaha, you sound like an over packer to me.
Let me just share what I bring for a day trip. (I swear Kaji's kwigy bo carrier doubles as a yorkie "diaper bag")
-snack sized sip lock baggie with kibble
-bottled water
-travel collapsible bowl (it can be folded up real small. I got it at Dollar tree)
-squeaky ball/plush toy (whatever is his fave at the moment)
-sweater
-poop bags (also a dollar tree purchase)
And of course he already has a car seat in the car, and he wears his harness before we even walk out of the door.
You might consider leaving a few things at your grandma's if your a frequent visitor. When I was on my own, I spent every weekend at my mom and dad's house. I bought a separate set of bowls for their house, an extra bag of food, a bed, and a few extra toys that we could leave behind.
For your little one, you might want to leave an extra expen/crate so your not lugging it around frequently. It gets annoying after a while.