A few games I've also added extra features to so they're more accessible (usually infinite lives/continues, nothing you couldn't get without a simple cheat code). But a handful I think might actually be interesting to other people. For instance, adding semi-rapid-fire to 1943 NES, start with full health (instead of a measly 30) on Metroid, speeding up that damn intro text on Bad Dudes NES.
It's been pretty satisfying figuring out how to change games with nothing but a disassembler and hex editor. As soon as I figure out how to post IPS files here I'll share a few of them.