I'd fiddled with numerous games on my PC as a child, but I suppose my first true modding experience was with Doom and Doom 2.
A friend of mine and I modified WAD files, edited sprites, and numerous other things.
One of our "shining achievements" was crafting a Christmas Mod for the game. We'd reworked every character sprite to make them look as "christmas-y" as possible[footnote]Complete with snow suits, boots, winter hats, etc.[/footnote], edited every fire-ball sprite to look like a snow-ball, redid many of the level textures to look snow-covered or candy cane colored, and changed the sprite-objects to make them look like pine trees, presents, etc.
One of my favorite bits was the changes I made to the death sprites. Whenever you killed a monster it would shrink down and turn into a gift. Was quite funny to run around through a scene of carnage only to see gifts strewn about instead of dead bodies.
Took quite a while to do, but we finished it in time for the Christmas holiday and shared it with any friends and family that played the game.