You should definitely include OOP basics and array manipulation...
Those are (at least in my opinion) the most important concepts that I didn't pick up right away...

Another useful thing to know about would be transformations.

Definitely, I'm thinking OOP basics - Encapsulation, Inheritance, and Polymorphism - are going to be pretty high on the list. They are concepts that I had to learn over again when I started with Processing and it would have been a lot easier if I started with fundamental things such as those, rather than after I had been (trying) to program for a some time. Some of the other things I was thinking of were: 

Also to make programming easier to stick with, even when it is frustratingly annoying, and not doing what you (think) you are telling it to do, I think to contextualize the information in terms of graphics, interaction, sound, gameplay, and simulation is pretty important. Things like animating a character across the screen is far more interesting than simply programming a system to analyze and return information or something similar. And if they are interested the information and skills will sink in and stick.

I would make it by steps, as Processing allows this. I think, from the code I see from beginners here, that lot of teachers do it this way.
First, the application lifecycle: what is start() and draw(), how to use keyPressed() and similar, what are global variables, avoid hiding them with local variables.
Introduce base controls: if/else, for, while...
Then, introducing custom functions, to help reducing copy / paste. Idem for arrays!
And only then, progressively inject classes, as a way to reduce even more repetitive code. Use them as simple structures, then add constructors, methods, and only later, inheritance, polymorphism and other harder concepts.

Processing made Java procedural for a reason: it is easier to understand than jumping straight into OOP, and it reduces a bit the boilerplate code. One can do simple and nice sketches without OOP, but when starting to be more ambitious, the need for OOP will be more flagrant. Of course, exercises must be designed to follow the progression. Sometime, we see here assignments where we think "classes are a good fit for this" but the student haven't seen them yet.

I'd add orthogonality. It's a nice concept that helps to make good coding and it can be used to descend into particular basic issues of different programming languages (as passing by value vs. passing by reference in Java/p5).
Regards!

Ale · http://60rpm.tv/i