Power to gas, as you mention, would be sufficient. There are also some interesting battery chemistries in the pipeline, like iron-air batteries. How about we keep building renewables at a rapid pace until we actually have enough to need storage? Until then we have time to improve storage technology and build some electrolyzers (we'll need lots of Hydrogen anyway for industrial purposes).
What's the risk? It's not like we run out of wind and sunshine anytime soon. Please don't answer that sometimes neither the sun shines nor the wind blows, because we already know that we'll need storage for a carbon free system.
