HTML5 games! They're easy, quick to set up, the feedback is instant and they're fun to play! I've been making HTML5 games as part of short, two hour collaborative coding exercises at work and it's awesome. Here's a recent one we did based on Flappy Bird [1].

I made a few bootstrap scripts here [2] to make it easier to get up and running.

1. https://github.com/hoorayimhelping/Flabby-Bird 2. https://github.com/hoorayimhelping/HTML5-GameKit