The Meaning of Life, the Universe, and Everything.
Caves usually have bats inside. They are tiny passive mobs with no drop, added for atmosphere. I even thought it is a special effect, not a mob!
I suggest to add same kind of "tiny atmospheric mobs" to each biome, to make it more live and vibrant.
Bees are spawn in flower forest.
Seagulls on beaches and above ocean biomes.
Crabs on beaches.
Birds on all kind of plains and hills.
They provide no drop and small amount of experience.
We can go further with bees and make them neutral mobs and add them hives. Hives located on the trees. If you try to attack hive - they become agressive. But if you manage to take hive, you can bring it to your base, and bees will spawn from it compleatly passive to you. To spawn, there must be no less then 10 flowers nearby. You can farm honey from it (put shugar inside, like you farm bone meal via composter). I have no idea what you can do with honey - write your ideas in comments below. But it would be cool to add hives to your farm!