I don't know if any of you play Magic: the Gathering, but it's a fun card game where the basic rules are not that complicated, but there are literally thousands of cards with different rules and mechanics, and millions of interactions between the cards, so the entire structure of the game is quite complex. Anyway, a few years ago I wondered how large a finite number we could generate (be it life, damage, creature power, or whatever) using a single deck, without utilizing an infinite combo. I was able to get a number greater than \(2 \uparrow\uparrow\uparrow\uparrow n\) for a pretty large n (I forget the exact number, I think around 1700 or so), and I was pretty impressed with the size of it. (I know it's not a large number in googology, but for a card game you don't expect to see numbers that large.)
Well, other people apparently pondered the same question - one challenge was to find the most damage that could be dealt on turn 1 with a single 60-card deck (so tighter restrictions than I gave) and one Kaitlyn Burnell produced a combo that dealt about \(2 \rightarrow 40 \rightarrow 23\) damage, then improved it to a combo that deals \(2 \rightarrow 30 \rightarrow 36\) damage! The ridiculous combo is described here.
Not to be outdone, Russell Jones has just posted a combo that deals more than \(2 \rightarrow 5 \rightarrow 113\) damage! I wouldn't have believed that you could get more up-arrows than cards in the deck. The deck description is posted here. Obviously you need some knowledge of the game to understand all the interactions.
I know this won't float everyone's boat, but I thought it was a fun little game to play, and I was impressed with how far people took it.
Incidentally, another fun thing done with magic was to prove that it was Turing-complete: