The Shalom meditation has been a frequently visited blog posting. It is done silently and is a mantra type. " Sha" occurs on the in-breath, "Om" on the outbreath. It is Hebrew for peace and the meditation carries with it awareness of personal peace as well as that for the universe.
Peace is a powerful component of serenity.
Another aspect of serenity is an underlying joy which is not dependent upon the minute to minute life experiences.
In Hebrew Simcha means joy. It can be used as a mantra as well. Silently say "Sim" on the inbreath and "Cha" on the outbreath.
The gutteral sound "cha" is not easy for everyone. Like the spelling of Hanakah/Chanakah it might be useful to use "Ha" on the outbreath.
Of course it might even be preferable. "Ha" is ha! The association with laughter is not coincidental here. It can even be emphasized as noted with the exclamation point.
More importantly– try it and enjoy it. Both peace and joy are necessary ingredients in serenity.