Nothing, there’s no room. Unless we count that short and not particularly comfy cupboard as “sitting atop”.
A couple boxes of cereal, a can of cashews, and a bag of tortilla chips.
My cat surprisingly doesn’t get up there. He doesn’t really have a good place to jump from so that’s probably why.
My fridge has only about six inches between the top and a set of cabinets above. The only thing I keep there are the reusable bags I take to stores if I’m buying more that would fit in my backpack
The box with miscellaneous change and extra keys.
My tool box.
2 pitchers used for baby formula, which we usually forget to use.
Two roasting pans.
A water bottle.
1 flashlight.
An open box with cables and wires that we keep thinking we’ll figure out what they go to.
I’m the only one in the house who can reach ������
a variety of breakfast cereals (including oatmeal) coffee, coffee filters, a small variety of odds and ends that ended up there for whatever reason.
Couple of ice cube trays. Visitors like ice, but I have no use for it.
Net onion bags waiting to be knotted together for use as pot scrubbers.
A digital clock
Couple of sour cream tubs with plastic bags in them
Few odd-sized plastic tubs
This doesn’t count anything that has fallen off the back and is stuck down among the freon coils.
2 cans of dog food
Plastic container of miscellaneous dog treats
2 Plastic pour out containers of cereal
Package of light bulbs, minus one
In the small gap between the fridge and the cabinet I see a bunch of unknown papers. A box of AAA batteries fit in front of the cabinet. The doors just clear it. A jar of Momma Sans “energy” pills get moved side to side when the doors need to be opened.
In the garage is the garage radio, which used to be the kitchen radio. Also 3 12 packs of soda and a box of freeze pops not yet frozen. During the winter 2 or 3 coolers are stacked up there instead of the sodas. They get on the floor during the summer since they are used for weekly grocery shopping.
A pair of cat travel boxes. Keeps them out of the way but readily available. I mean the boxes are out of the way not that I keep the cats in them 
Several rolls of aluminum foil, plastic wrap, and parchment paper. A stack of wooden bowls. A stock pot. A breakfast sandwich maker. A cabbage tamper and a refractometer. Occasionally the cat - she uses it as a bridge to get to the top of the cabinets.
Oatmeal, a basket of kitchen cloths/dish towels, and a rubbermaid container of dry cat food.
I have two other refrigerators that I didn’t even think of including.
Basement fridge:
Microwave, circa 1984, still fully functional.
Plugged in battery charger and two batteries for cordless Dewalt drill.
Garage mini-fridge:
CD/radio ‘boom box’, circa 1990, still fully functional.
Stack of about seven baseball caps that I’ve accumulated that I never, ever, ever wear.
mmm
A pound of See’s chocolates. The only good thing about going to LA for the weekend was that I knew the wife was missing those. Had to look all over for the dark chocolate varieties, but it was worth it.
Thanks for starting this thread now – a week ago, my answer would’ve been a dejected “Nuthin’…”
You can order See’s right to your door…I mean refrigerator top.
StG
I was going to suggest one of those standing wheelchairs that you like so much!?!
What’s on our refrigerator (from memory):
One set of Good Knives, in holder
Black and Decker Food Processor, still in its original box*
Container of industrial-strength catnip
Some random stuff I can’t recall
*The Food Processor is something I’ve had for decades. I use it on a regular basis, but after it’s been cleaned, it goes right back in the original box. Unlike other kitchen gadgets, I never have to wonder where it is. And it’s still in great shape.
I still have a multiple-decades-old hairdryer in my bathroom closet, also still in its original box, and heavily used.
A wire basket napkin holder
The rest of the package of napkins (which really should be in the pantry)
A package of lemon Oreos
Two cans of Rustoleum spray paint
A glass beer mug full of paint brushes
A digital photo frame that hasn’t been turned on in 10+ years
A small china teacup and saucer
A package of peppermint candy canes
And when we buy a two-pack of paper towels, the one waiting to go on the spindle goes on top of the fridge.
One radio. One cat. She likes the fridge because she can survey her domain.
Patience the Labrador’s Frisbee is up there.
Sadly, there is no more Patience to chase and fetch it. 
An antique Sterling silver tea set on a tray at the front with plenty of space left over under the cabinet that fits a kitty very well.