This is tough to answer without knowing more about you. What skills, resources, and relationships do you have at your disposal?

One of the simplest businesses to start is a consulting practice. However, "immediately" generating income will depend heavily on what sort of relationships / contacts you already have, and how great the need is for your particular expertise.

Since all I know about you so far is that you've been involved in a startup, I'd recommend you look to your local startup / entrepreneurship community to find another company who needs some short-term work you could do on an hourly basis. You've apparently got surplus time, so monetize that while you figure out what's next.