Ya now that game controllers have become so cheap I usually snag a couple from garage sales in case I run into a project that could use it. Also If a buddy upgrades his phone and no one can use his old one I save them for the same reason. I'll use the Processing platform, since I've been using it for years, to basically write a user interface app to put on the old phone and use it's wifi to control stuff and display data. But processing only works with android, PC, and raspberry PIs. I don't think it works with Iphones, but same programming language as arduino.