Under the Stairs Pantry Ideas?

I have a cubby closet too, and I have all the board games and crafts stored there. It stays dry and cool, so the crafts don't get damaged, and the kids get in and out more easily than I can so it made sense to use a little space for little people.
We also have a small amount of cabinets, so I bought a pantry cabinet that matches our kitchen and put it in the mud room for canned goods, pasta, trash bags, snacks, etc. Saves on kitchen space.

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I would try to use the shelf space you have for bulky items that are used infrequently... like big appliances: crockpot/rice cooker/ griddle/ rotisserie/popcorn popper... that kind of thing. If you have a pet, it might be a good place to keep a rolling container for the pet food. We have one we use for our dog's food that holds 30 gallons with a snap shut lid and wheels... so I roll it out into the kitchen to feed her and roll it away after. You can hang mops/brooms/cleaning buckets/ironing boards from the wall.

And if you have any high shelf space (too high for stuff that you don't want to pull down on top of yourself).. you can stash mega packs of paper towels/toilet tissue/napkins up there. Then if they fall... it doesn't hurt, lol.

If you have a blank wall inside, I have a friend who used the chalkboard paint to pain the inside of a closet wall. Your 3 year old might enjoy having a space like that to draw pictures on (or you could keep a running inventory or grocery list on it?).

IKEA in Canton has wonderful storage stuff - inexpensive too.

Since it is kinda hard to get into...I am thinking cleaning supplies or extra bulk paper products, cups,plates napkins T.P, trash bags, extra soaps, etc..that kind of stuff?

I would get a big storage container on wheels that could go in and slide under out of your way. I have one of those that I keep all the tupperware / gladware type containers and lids in. I gave up on trying to keep them all put together and now just toss them in and close the lid. they are there when I need them but not sliding to the back of cabinets that I can't see. that might also be a good space to put your winter boots and shoes etc out of the way but you again will know where they are.