Interactive Protyping software

We currently use PowerPoint to mock up screens.

We would like to use something that allows us to rapidly build a screen and show what the interaction between the user and screen would be; and then let a stakeholder play with the screen before they give us the green light to build it. 

We have not decided if a developer person would use this software or if a non-technical person would. We are currently leaning towards a non-technical person (the person who does all of our training materials, eLearning, etc, so they have a good background in buulding training modules/screens but not interactive). 

Any suggestions for software, to buy or open source, that can do this?