Old games are great on their own but I've always been a fan of cheating and messing around with codes (like Big Head Mode for example) so when I first found out that most emulators had their own cheat search to make what you want I jumped on it. However at some point I realized I was incredibly restricted in what I could do and merely cheating can get tiring. That's when I learnt of Lua scripting.
Lua is basically code and in Emulators can potentially be used for amazing things like the jetpack mod for Super Mario Bros or the various scripts for Pokemon games that help you beat the randomizer and get shinies. Although I'm still a complete noob in comparison.
So far I've only made a script for Super Mario that says what powerup and a script for Sonic Advance that can do really random stuff like swapping the character every frame and changing your color every time you pick up a ring. I'm finding it quite fun even though nothing seems properly sharable yet.
Feel free to suggest a retro game from consoles like NES,SNES, GB-GBA for me to mess with and maybe some ideas of what to try doing too.
P.s. I have an old 10+ year old laptop now so I can post more often.