Ever since Minecraft 1.8 the community invented redstone modules that could be imported into your game just by copy+pasting some text into a command block to build a self-building contraption with mod-like capabilities. Since then they have only gotten more efficient, more complex, and less laggy. I thought I'd use this thread to post a few of my favorites and encourage others to do the same.
Dynamo by Chocoparrot
Minecraft has a lot of structures to explore, but once you've seen one village you've seen 'em all. This adds a whopping 30 new biome-specific custom structures to randomly generate in your worlds. Ice Spike castles, minotaur towers, and all sorts of things.
This one is downright ridiculous. It's less like a mod and more like a modpack, Magic Machines, Magic Staffs, Custom Structures, More Creatures, Spirit Gems, Ascended Enchants, all honed after years of trialing different ways of going about it. Each can be installed individually.
Not all of these have to be so huge and ambitious. Sometimes a small change winds up being the one I use the most. Nothing's quite as frustrating as having all your items burn in lava or get scattered widely at the bottom of the ocean. This stores your items in a simple gravestone that you break to get them back. I've seen a lot of takes on this concept but never one so clean and effective.
Been seeing views on this one but no posts! I figure I'll toss in another example: Tyruswoo's Structure Generator. Particularly special in that, unlike Dynamo, it is customizable. If you can stick a structure into a structure block of your choosing, you can make it randomly generate just by slotting it in.
I will emphasize that these are quite simple to input into your worlds. Some as simple as just putting some command text into a command block and activating it, some a couple of extra steps (like the tempest box has a resource pack), but generally a heck of a lot easier than wrestling with modding, for many of the same benefits.