More formal:
* Whisper: This is the most direct synonym for "say quietly."
* Mutter: Implies speaking in a low, indistinct tone, often with a hint of disapproval or frustration.
* Murmur: Similar to "mutter," but can be more neutral.
* Speak softly: This is a general way to describe speaking quietly.
* Subdue one's voice: This emphasizes the act of controlling the volume of one's speech.
More informal:
* Hush: This is a more casual way to say "be quiet."
* Don't let out a peep: Implies absolute silence.
* Keep it down: This is a direct way to tell someone to be quieter.
* Speak in hushed tones: This emphasizes the secrecy or intimacy of the conversation.
Figurative:
* Hint: This implies speaking indirectly or suggesting something subtly.
* Confide: This means to share a secret with someone.
The best alternative will depend on the specific situation and the desired effect.