Je t'embrasse (pronounced "zhuh tum-brahs")
This literally means "I kiss you" but is often used as a way to say goodbye in a more affectionate way.
You can also say:
Je t'envoie des bisous (pronounced "zhuh ton-vwa deh bee-zoo") which literally means "I send you kisses."
Bisous is a more informal term for kisses.
The choice between the two depends on the context and your relationship with the person you are talking to. "Je t'embrasse" is more intimate, while "Je t'envoie des bisous" is more casual.