Sillinesses are the things that are silly, or not serious.
Sillinesses are usually fun and make people laugh.
Examples:
The children's sillinesses made everyone smile.
She enjoys the sillinesses of her friends.
We laughed at the sillinesses of the clown.
The sillinesses of the dog always make me happy.
The sillinesses of the movie made everyone laugh out loud.
The sillinesses of the party will be remembered for a long time.