You don't see a functional difference between "Enjoy!" and "Be happy."? You're presumably happy or at least happier while you're enjoying something, but that doesn't mean they're not functionally different. "Enjoy" pertains to how you relate to a particular thing or activity. Being happy has to do with your mood and state of mind in general. It's not about any particular activity.
I, too, thought the literal translation "Enjoy yourself" should be accepted, but apparently that just doesn't fit with what it actually conveys in the German language and culture. Every culture is different, and literal translations can be wrong. "Viel Spaß!" is closer to "Enjoy yourself," although I think the usual translation is "Have fun!"
To be more precise, Google currently translates "Freuen Sie scih." as "You can look forward to it." That's disappointing. Google is not usually that far off. As you know, Google Translate is a useful tool, but not an authority. Fortunately, Google got "Sei glücklcih" right.