Talk to Cima inside the Lakeside Stable. She wants to know what's going on with lightning constantly striking the stable.
If you climb up to the rooftop of the Lakeside Stable, you will see an axe. Pluck it off the horse's head. If your inventory is full, you will have to let go of something to get the axe. You can come back for it later. If it's raining, you will probably need to use Revali's Gale to climb since the surface is wet, or you will have to wait until the weather becomes clear.
Return to Cima and tell her what happened and she will give you the Rubber Helm, one of the pieces of the Rubber armor set.