I would link to the Github, for sure, unless your github is mostly all just hobby stuff that you don't necessarily do your
best work, but even then I think that would be good.
I would present any screenshots of any ui, even if you didn't design the ui, you can say "but i made this thing work"
I have only seen developer resumes, not portfolios, and they listed languages of course, but also things like design techniques or methodologies they've followed, classes taken and certifications received, and also user groups they attend. List any open source projects you've contributed to, or even just repurposed toward some goal (and describe what you did to it to make it work for you)
Now, you might, if you have some particularly complex object that you're proud of, maybe perhaps create a Processing sketch or similar data visualization type example, and perhaps post screenshots of that.
But outside of that, it is ultimately intangible, and I think you have the right idea. I have not seen portfolios per se, but I have seen lists of these kinds of things. I've also seen employers ask for snippets of code.