For a tank that size, you're looking at snails, shrimp, and nano fish like chili rasboras or celestial pearl danios. Fish under an inch-- I'd personally stay smaller than neons. Or a betta or a dwarf puffer.
For plants, the easy small plants for me are anubias, java fern, wendtii crypts, maybe dwarf sag if you're okay with it growing tall enough the tops float along the top of the water (I have some in my 5 gallon). Some stem plants like ludwigia, so long as you trim them as they reach the top. Java moss or subwassertang is nice--I prefer the subwassertang because it's not as messy and isn't so light it floats all over the place.