Sugarcane Farm

Sugarcane just needs to touch water and stand on sand, dirt, or grass. It doesn't care about light level, and it grows up to three blocks tall on its own. The one wrinkle is that it has to be broken to drop anything, so an automatic farm needs a way to snap off the top without disturbing the base block, since the base is what keeps growing.

The fix everyone uses is an observer watching the second block of the stalk, wired to a piston next to it. Grow into the third block, the observer fires, the piston knocks the plant over, and everything above the base pops off as items. The base survives untouched and starts again immediately.

Scaling this up is just repeating that one unit down a canal. Plant sugarcane on both banks, one observer and piston behind each stalk, and run a hopper line or a hopper minecart underneath to catch the drops. No redstone clock needed anywhere, since each column fires on its own the second it's ready. It's one of the only fully automatic farms in the game that's entirely observer-driven with zero timing components.

The only real decision left is collection. A single hopper works fine for a small patch. If you're feeding an enchanting operation or a book economy, run a minecart down the middle instead, since trying to funnel a long row into one corner hopper gets slow.

This tends to be the first automatic farm most people build, and it stays useful the whole game. Paper never stops mattering for maps and enchanted books, and sugar is one of four ingredients you can't get around if you want cake.