Formal:
* Shabbat Shalom to you as well. (This is a direct and polite response.)
* Shabbat Shalom to you too. (Similar to the above, but slightly less formal.)
Informal:
* Shalom! (This is a common and friendly response.)
* Have a good Shabbat. (A simple and sincere wish.)
* You too. (This is a casual and appropriate response for someone you know well.)
Other options:
* And a good Shabbat to you! (This is a bit more emphatic.)
* May you have a restful and meaningful Shabbat. (This expresses a personal wish.)
The best way to respond depends on your relationship with the person saying "Shabbat Shalom" and the tone of the interaction.