- In desert biomes, have plateaus, tumbleweed, and some minor rock formations(maybe stash some snakes under some of them).
- In forest biomes have the trees sway slightly with the wind and make the leaves audible when this happens, pine cones for the pine trees.
- In tropical biomes have palm trees w/ coconuts(possibly break them and drink the milk), high and low tides, waves rolling in and out.
- In taiga biomes, have yetis(make them extremely rare), icicles hanging off houses, etc.
- For caves, have water droplets fall from the ceiling(don't over do it), have all your actions(walking, mining, etc.) echo throughout the cave, stalagmites and stalactites.
Let me know if these are good ideas and if they are, i'll try to make a mod for them.