Formal:
* Optional
* Voluntary
* Discretionary
* Non-compulsory
* Elective
* Permissive
Informal:
* Up to you
* Not required
* Not a must
* Not necessary
* Free choice
More specific:
* Recommended: If it's suggested but not required.
* Suggested: Similar to "recommended".
* Optional: If it's a choice, but not a requirement.
* Elective: If it's a course of action that can be chosen but is not required.
The best synonym will depend on the specific context and the desired tone.