How can I use sunflower seed shells?

Use them to make a picture. Draw or trace a picture. Then stick these and other nature elements to make a unique picture.

compost them.

Yes, they make great compost!

Mix them in your soil to break up clay

Use as mulch

You can add them to your compost pile

I'd put them in the compost.

I would say compost pile. However ,if they aren't covered with bird poop you could make art with them. Google sunflower seed art.