What do programming novices need to know when using MatLab?
With only 5 sessions, I would abandon the idea of building something cool, and focus with laser-like precision on whatever they need to know going forward in their coursework.  There is such a small amount of time to learn the basics of any programming language.

MatLab supports lambdas, objects, and GUI design, but unless they are going to get into rather serious programming, I imagine that you would steer clear of these.  (I don't know how you would cover any of them in 5 days with novices in any case.)

The danger in heading towards UI and a game is that they will come out without being able to do what they presumably need in the next class, such as matrix multiplication, or graphing an integral multiplication.  You may be better using your five sessions to delve into five areas of greatest need, as determined by the future required coursework of your students.