I think they would make a good surface to block knitted items on. Some people use those giant puzzle pieces made of a similar material as a blocking mat. Something similar could probably be built from those.

They could be used as insulation for e.g. in a cold frame where plants are being overwintered.

Nothing crafty, I just thought using the flat boards to sit on the floor or on the grass (if you don’t want to get your clothes dirty) Or they can be use when working in the garden, so you can step/knee on the soil easily.

About the ones that are like giant haircurlers, I thought of cuting them, so you have many flats circles, and make christmas ornaments.

I like the sitting-on-the-grass idea! you could make little pillow-case inspired covers so you wouldn’t have to sit on the gross peely old board. you could get pretty crafty!

You can suggest they get made in compostable material - then you can just chuck them in a compost heap in small strips. Visit link and ask if it’s doable!

Archery targets
boffing weapons ( a game kids play-they make swords sheilds etc from pool noodles and foam )
giant puzzle pieces for kids
cut into foam stamps
knee pads for gardening or whatever
cut into cushions and cover for outdoor furniture
padding for the ground around swingsets?

oh, bulletin board?

I use a swimming board as a macrame board. The T pins fit in so well! You could see if a local middle school is doing some macrame projects (maybe friendship bracelets). They could use all of the old boards :-)

The best use I’ve seen for these things is to make Halloween Gravestones props out of them.
Use a razor blade, stencils, and faux stone spray paint to design whatever you want, then insert stakes for great yard decorations!

http://www.madhauscreative.com/one%20dollar%20graves.html

http://www.virtualplacebo.com/halloween/props/graveyard_stones.htm

http://halloween.home2garden.org/make-a-halloween-gravestone/

They would insulate incredibly well - putting them around a hot water tank would help it get warm and stay warm, saving fuel and therefore money and the environment.

If you had enough you could use them to do the loft too!

i save all “expanded” products like these (and the obvious styrofoam packing/cups/etc.) and repurpose as insulation in my basement and attic. relatively large flat, stiff shapes like these can be good for filling specific holes, whereas most styrofoam i end up breaking up into smaller popcorn pieces and filling cavities.

I’m an ex-swimming teacher. we would cut them up and string them to make the floating dividers so that classes wouldn’t bump into each other.

Make a lifejacket
I found two discarded ones on the beach so I made them into a lifejacket, for my kids, using an old raincoat, or old swim suits. I cut up the coat into the shape of two vests added binding and tied them at the sides, so you have one float on the front and one on the back.