When I was young I was obsessed with weapons. As many nerdy children and teenagers do, I had a particular fondness for medieval weaponry, but I also loved guns and explosions and lasers too. One of my prized possessions in my youth was a standing chalkboard on wheels. (Bet you didn't expect me to say that, right? You were totally expecting me to say "pocketknife" or something.) On that chalkboard I devised all manner and number of games and activities. My favorite was a game that I created to play with my little brother, The Explosives Game. (I'm guessing I was around age 8 or so at this time.)
In this game I would create scenarios similar to how a Dungeon Master would in D&D, and then my little brother, armed with a number of sticks of dynamite and barrels of TNT and so on, would play the part of a guy who just wanted to blow things up and kill people. The backstory of his enemies was always rather weak, as is always the case when little boys play with their GI JOEs and the like—they were "bad guys"—and that was justification enough to wander into their homes, kill everyone inside, loot their armory, and destroy every possession that they held dear. This is the sort of thing that I imagine gets you on a domestic terrorist watch list these days, but I assume that any number of creative children come up with similar games of wanton violence. Over the months (years?) I established quite an extensive (you know, for a kid) core handbook for this game, though the actual gameplay was flexible enough that much was made up as I went along so that whoever I felt like the winner should be would in fact be declared the winner. (I think I tried to split play sessions 50/50 between us.) I had a black binder filled with page after page of stats on explosive devices: what they looked like on the board, their explosive radius, how much damage they did, how much they cost at a shop, and so on. It was like the Player's Handbook and Monster Manual for my magnum opus. I would enjoy having that book now, though it's possible I destroyed it at some point; I vacillate between treasuring such nostalgic memories, and wanting to erase all traces of my past foolish low-quality creative endeavors. (Some of my Photoshops from a decade ago make me want to throw up.)
Of course that wasn't the only game I created, but it was the most dear to my heart, and it was one of the few that I ever wrote down rules and numbers for. Probably in second place was The Great Race, which basically consisted of putting a dozen or two small toys (especially Happy Meal toys) in a row on the linoleum kitchen floor and then racing them to the finish line using a completely arbitrary and made-up series of rules. In one game my box of fries had a special power that allowed him to jump ahead of any car on the board and my pony got to move twice every turn, and in the same game maybe my brother's milkshake got to fire missiles at my racers, killing them and removing them from the game. It didn't make any sense, but then again neither does Super Smash Brothers, and even adults like that game for some reason.
I think I turned out okay.