2x4 scrap ideas?

I'm losing a major chunk of my storage space and I need to rearrange and down size a bit over the next couple months.

Is there anything I can use small 2x4 scraps for? Like 15" or less? Kindling?

I'm trying to prioritize what to keep...

Firewood certainly. The size you mention would temp me to try to build shelves.   If one considers building on 16” centers, that size might be good for bracing.

Chunky birdhouses? Only half-joking.

You could build a lot of stuff with small cut off lumber. I just built a stool for the pep badge with barely longer pieces

I often use them to make jigs, templates, shims, similar things. Having a bunch of cut off scrap is fun. Check out YouTuber "Pask Makes" scrap wood challenge for more inspiration.

If you know of someone with a table saw those 2x4 can be made into 1x4, 2x2, 1x1, etc.

Here are some things you could make with a 2x4, 1x4, etc.:

https://permies.com/wiki/124795/pep-dimensional-woodworking/step-stool-PEP-BB-dimensional

https://permies.com/wiki/99532/pep-dimensional-woodworking/Basic-shelves-PEP-BB-dimensional

https://permies.com/wiki/124803/pep-dimensional-woodworking/Simple-open-toolbox-PEP-BB

https://permies.com/wiki/124800/pep-dimensional-woodworking/wood-box-crate-PEP-BB

https://permies.com/wiki/148454/pep-dimensional-woodworking/Build-Water-Tight-Wood-Bucket

https://permies.com/wiki/99526/pep-dimensional-woodworking/sign-PEP-BB-dimensional-sand

Short pieces of 2 x 4 lumber are ideal for making adobe brick forms. A 2 x 4 that is 15 inches long would enable a maximum length of 15 inches when other scraps are used as end caps. When ends are capped with other 15 inch pieces, your maximum brick size would be 15 inches long by 12 inches. This would be a very heavy brick! You could divide your 15 inch length in two using a middle stud ( an H-shape), then cap the ends and you'd get 2 bricks each pour that are 6 x 12. That's a nice shape for small projects. I like bricks that are 4 x 10 x 13. I also make trapezoid shapes for curves.
Just dunk the form in water, fill it with a wet dough-consistency mix of 10% clay and 90% (sand, straw, dry clippings, stickers whatever), slip off the form and you have your first brick!
For more info on entire cities built this way, see:
https://en.wikipedia.org/wiki/Adobe
https://aces.nmsu.edu/pubs/_g/G521/

I get scraps that size a lot.  Most of mine get made into biochar.  They also work great for being made into puzzle boxes, cut into smaller pieces for birdhouses and children's toys, crafts, that type of thing if you enjoy doing it.  I have too many projects already, hence the biochar.

Time to learn to whittle/carve?

I preface this by saying I'm no carpenter.
I rely way too much on screws and I'm building nothing more critical than a chicken coop.

That being said, use them extensively for  corners, legs, "hurricane" ties , "beams",  and "posts".

When I'm building something box like, which I usually am, I find that a chunk of 2x4 makes a substantial corner.
This is in contrast to the 5/16" thick pallet deck boards that I use for the sides, bottom or top.
By extension, they also work as legs for benches, stools or low tables.

When building a shed roof  I screw the 4" side of a 2x4 to the face of the top plate at regular intervals and screw my rafters to the 2" side.
They are basically hurricane ties, but  made of 2x4.
I leave them long enough to extend past the top of the rafter, then I cut them flush with the rafter with a sawzall.

To create a kind of beam or post  I will sandwich short chunks of  2x4 between between longer 5/16" thick pallet deck board.
This can be combined with the hurricane ties, alternating between horizontal or vertical chunk's of 2x4 as needed.

By overlapping two chunks of 2x4 I can make a longer "post" that is roughly "4x4".
This is most useful when I have  eight foot long 2x4s and I need a foot or two more in length.