Formal:
* Diga alguma coisa: This is the most direct translation and is appropriate for formal situations.
Informal:
* Diz alguma coisa: This is a more casual way to say "Say something" and is appropriate for friends and family.
* Fala alguma coisa: This is another informal option that can be used in a variety of situations.
Other options:
* Diga alguma coisa, por favor: This is a polite way to ask someone to say something.
* Pode dizer alguma coisa? This is a more formal way to ask someone to say something.
The best way to say "Say something" in Portuguese will depend on the specific context and the level of formality you want to use.