Another idea along the same vein is temperature control. Say the comfortable range is 20 to 25 degrees. Normally air conditioning in the summer cools to 25, once electricity becomes free it could turn on and start cooling towards 20, storing that power in the form of cool air. In the winter, buildings with electric heating could do the opposite towards 25.

These systems spin up quickly (at least in residential homes) and are a substantial fraction of our energy use.