You can combine your demo and onboarding process into one thing. My favorite example of this is NewsBlur: https://newsblur.com/ (Hit the "Try out NewsBlur" button)
Basically, drop them into a demo of the product with pre-filled data. They can make changes all they want, and if they want to save it, they'll need to register an account.
Basically, drop them into a demo of the product with pre-filled data. They can make changes all they want, and if they want to save it, they'll need to register an account.