What is the best corporate structure for a for-profit spinoff from a non-profit that is going to be wholly owned by the non-profit?

Depending on what your objectives are, you can really get into the high weeds with esoteric things like a Foundation controlling both entities. But for basic simplicity, a standard C corp makes a lot of sense. Talk with a good tax attorney on the intercompany issues (e.g., debt, interest, capital structure, contribution limits, accumulated net income of for-profit sub, allocated costs, etc.).

You may want to consider a single member LLC. We set one up for a Real Estate Venture where the single member was the Foundation. The LLC provided the legal security for the Foundation and the profit (loss) flows to the member is a single member entity. Just a thought.