Ah, yes, well that would be, wouldn’t it? :o
I routinely buckle-in bags of groceries, pet carriers, etc.
For a long time in the early 90s, I drove around with a Mazinga Shogun Warrior strapped into my passenger seat. (Yes, I’m a dork. Hi!)
Alcohol and watermelons and the like. I was a boyscout so I’d use anything at hand to make some work. Adapt and overcome.
-Cat in a carrier
-Cakes and other baked goods. I’ve strapped brownies in before too 
(depending on what or who else is in the car.) Now that someone mentioned there’s room for beer in the back seat on the floor I guess a cake container would work there too.
-Flowers (when hubby would send flowers to work, I’d put the vase in a copier paper box and strap it in)
-Computer stuff (monitors, towers)
-TVs
-Fragile stuff in a box
I always feel weird when I have strange stuff strapped in, like the cakes - because in case anything ever happened or I got pulled over someone would be like, ‘wtf?’
I created a seatbelt attachment for my cat Lenny, based on one I saw marketed for dogs. He used to get car sick if he was in a cat carrier. The cat seatbelt used a small dog harness (that went around the chest and arms, not around the neck at all), and then a a loop through which a seatelt is attached.
So he could sit up and look out the window, but he was prettymuch confined to his seat. In the event of a collision, the harness would work as well as a human sealtbelt with a shoulder strap.
This past Thanksgiving I ordered a fancy-ass free range organic turkey. (It was marvelous, but was it the brine or the bird?) They couldn’t tell me ahead of time how big they’d be, as they were roaming free in the field when I ordered mine, but I picked a ‘medium’. When I picked them up they apologetically told me that the turkeys had been big this year and they gave me the smallest medium, which was SEVENTEEN POUNDS. (And then people didn’t show up who were supposed to, so it mostly went uneaten. But it was incredibly good.)
Anyway, my car “put a seatbelt on!” beeper kept going off until I put the seatbelt on the turkey, because I guess it thought it was a child - there must be a weight sensor in there. It’s never gone off for other heavy stuff, like kitty litter or anything, but it beeped and flashed for my giant turkey.
In my old car the seatbelt was just tight enough to hold a large coke from McDonalds (in a cup holder) steady on the drive home.
My new car has never been to McDonalds with me as the owner.
Sure I do. The deer around here require you to stop quickly. I put what I can on the floor and surround it or it gets belted in.
This reminds me of the Doper that had paint all over the interior of a car and on them after an accident.
I’ve belted in computer towers, monitors, coolers and duffelbags. The last two while camping.
I put a seatbelt round my hamster cage and also one of my stuffed animal toys!
Like everyone else, I’ve belted in lots of inanimate objects, pets and plants.
Let’s just say you’d have to be a damned fool not to belt in a 5 gallon glass bottle filled with Mead. That’s not something I’d like to have to clean up in a car if something happened to it.
It was take-out food, and I promise to never, ever do it again.
I’ve got a harness for my dog that is made to be attached to the seatbelt. Animals really should be secured in some fashion, and her crate doesn’t fit in my car when assembled.
But the oddest thing I’ve belted in was a ~5L liquid nitrogen dewar. It didn’t fit well on the floor, and they can’t be closed securely (lid must be loose to allow for off-gassing). I had a great time imagining what it would have been like if it spilled all over my car, or how I would have explained myself to a traffic cop.
On pumpkins. Everytime without fail I forget to bring the truck when pumpkin picking and all I have is a back seat. So I buckle the big ones in because I don’t want a repeat of 92’. When I broke what was supposed to be the Great Pumpkin at a harvest festival because it wasn’t buckled in. 
I always buckled my Parrot ( in her cage ) in when I took her for rides. She loved to ride in the truck.
2 corgis
+
cheescake (in box)
Nothing so far, I did transport a computer monitor but the contours of the back seat held it enough so that I could drive around town with it face down and not moving too much.
I’ve belted in my tackle box while out doing a bit of local pond hunting where I like to be able to move fast. It’s a pretty big tackle box though, not just the standard tool box sized thing.
I’ve also buckled in alcohol. Huge cases of beer. I’ve also attempted to buckle in a keg but it wouldn’t work and the keg eventually had to be moved to the trunk. I buckled in stuffed animals because it makes my niece laugh.