Maybe a low plastic locker would fit under there so some of that space can be used. Nobody is going to want to stoop under there to pull out things. Even if you floored and roofed it it is not going to be a space that's comfortable to use.

I'd buy a shed. It isn't tall enough to really store anything under there, plus it will be damp and gross.

It looks like your deck seen better days and could use re-decking. While doing that install under deck drainage system and you will have dry space under there and you can store whatever.

Good luck

And if you can get that area dry, you could build a pull-out something to go under the deck.

But for tha tmatter, that little alcove of the patio between the stair and the house doesn't look like people use it for sitting--it could be a place to put some sort of storage units. Like a rack for bikes and scooters, and maybe a plastic storage bench, or a half-height storage "shed"

A bike or two could roll under the stairs from outside.

That space is not good for storing much other than a few unused flower pots, if you are so inclined to store unused flower pots.

Do you actually use the bicycles? Or, do they just get stored?

Are you talking about the kids toys, or adult bicycles?

It looks like you would need a better solution, such as a small storage shed, to put your bikes.

How ever would you roll a bike under that deck?

That space offers you nothing that you would want from a storage area. It looks as if it could harbor critters and it looks as if it is prone to being wet.

I have seen some backyard fixtures from Rubbermaid that might serve your purpose for the lighter weight items.

That little battery powered Jeep really can't be left in the elements. And, it would be a little awkward to lift in and out.

Maybe you don't really need a storage shed, but, rather a type of garden trunk or box

Give Rubbermaid a google. I know that they make some rather large ones to keep outdoor cushions and such.