Simple:
* That's a silly idea!
* She made a silly mistake.
* He wore a silly hat to the party.
Descriptive:
* The clown's silly antics made the children laugh.
* The book was filled with silly jokes and puns.
* The movie had a silly plot, but it was still entertaining.
With feelings:
* I felt silly wearing my pajamas to the grocery store.
* He looked silly trying to ride a unicycle.
* It's silly to be afraid of the dark.
Figurative:
* "Don't be silly," she said, "I'm not going to eat all the cookies."
* He made a silly excuse for being late.
* The news report had a silly tone, downplaying the seriousness of the situation.
Remember: "Silly" can be subjective, so the context of your sentence will determine whether it's used in a playful, critical, or even sarcastic way.