What about having worms(critters) actually move in dirt blocks, like the truffle worm, but passively and slowly. Would need to set a spawn chance.
Some bees as well, actual, non aggressive bees. Could spawn near nests in the jungle, and rarely near flowers on the surface.
A way to make birds sit on trees, like actual birds! And fly away if the tree is cut.
More insect wildlife in grass, like mantis staying still hidden within, or beetles on trunks.
Small spiders spawning on placed cobwebs, with variety(there's soo much possible).
Mosquitoes during dusk and night near water.
Dragonflies over water too(except ocean).
Sand ticks jumping around if there's movement and waiting in the sand.
Baby granit golems hiding as rocks on the ground.
A magnificent camo moth that stays on marble walls to become almost invisible.
Do I need to continue, or is this enough to work until next year?
edit: oh, and small spirits floating around in the respective soul biome.