The adjective for "sphere" is "spherical."
You can say "a ball is round and spherical" but I don't like it because it is redundant. "Round" can mean spherical. (It can ALSO mean "circular.")
In a geometrical context, the opposite of "flat" is "solid." Balls and blocks are solid objects.
A block that is the same size in all three dimensions is a cube, and the adjective is "cubical."
It is quite hard to describe things precisely in words because most of these words have overlapping meanings. For example, a coin and are both "circular," because the word circular can more than one meaning.
This is how I would personally describe these objects:
A coin is a flat, circular disk. ("Disk" tells is that it is NOT a ring.)
A ball is a hollow, inflated sphere.
A block is cubical.
A table napkin is flat and square.