If it's a game engine, I tend to do a simple port of my Proximity game[1] to it. I don't usually finish it enough to release it, though (I probably could and should in retrospect). I've done that for Flash, C#/XNA (eventually released on Xbox Live Indie Games as "Proximity 2"), J2ME, Java/Kindle, iOS/Objective-C (eventually released, but no longer in the store), Python (just a command line ASCII version), LUA/Pico-8 (nearly done, I just need to force myself to spend a couple weekends and finish it up), PlayDate (functional, mostly a port of my Pico-8 version they are both LUA-based, but it's not pretty and I don't have my PlayDate yet for testing to feel comfortable releasing the game yet), C++ (mostly made this for my portfolio for game dev jobs), and probably a couple others I'm forgetting.
Currently working on a new version in C#/Monogame, and next version after that will probably be Unity so I can make a VR version, something that's on my bucket list. Also at some point I'm going to make a new web version again using Javascript/Phaser (already made another simple game in Phaser, so it shouldn't be too hard, just need to actually do it).
It's been going slow though since I have so much less spare time and energy nowadays, though.