Well obviously you have to know the skill level of your target audience. I can say that someone who is just learning programming is probably not going to be picking up React as their first tech when they don’t even know basic JS. Next you identify the strengths of the tech you want to showcase. If it is good at fetching and caching, then gear a project to show that while talking about maybe why it wouldn’t be the best for a todo app.
Now given that info, you probably can target something a little more sophisticated. Maybe a small and minimal chat app. Maybe a weather app, maybe something that is tracking data in real time like a twitter feed or a feed of some other kind. You could do a side project tie in with what @Corobori and his mapbox project.
Last but not least…
Shameless plug incoming! Maybe you could take a look at my book “The Programmers Idea Book” and implement one of the 200 project ideas in that.