My favorite mod, Smart Moving, hasn't been updated in quite some time, it's still stuck on version 1.8.
It's a mod that gave you quite a few more actions that fit in very well with the style of the game, especially the exploration aspect.
It allowed you to crawl to fit through one block spaces, grab onto ledges 2 blocks or taller, and everything in between. Even some more fun aspects like diving, sliding, and wall jumping.
I've seen a lot of mods around that allow you to change your characters animations pretty easily, like Quark and More Player Models. Obviously animations and actual hit-boxes are two different things however, but hopefully it's at least a good starting point for someone?
Thanks for reading.