What can I do with leftover hardwood flooring pieces?

store them as you found them in the store ie were they laying flat standing on end? you may need to replace / patch the floor at some point...usually after the flooring has been discontinued.

I usually have some of these around. Come in handy if you have the tools to work them into smaller width or thickness. I glued some cutoffs from my closet floor together making a single-sided cutting board for the kitchen at work. Single sided because the back has grooves. If you do not have the equipment to cut, thin, glue and shape for smaller wooden projects, donating to Habitat for Humanity is a good idea.

depending on how much you have, save some for patch repairs and then maybe make planter boxes with some of the extra?

bird house, table top in a herringbone pattern picture frames wall hanging

Depending on how much you have left, maybe do a feature wall. You could sand the boards and finish them with different colours of stain.

Be sure you keep some for future repairs that you may need on your flooring.