You can say "Howdy." It's a shortening of "How do you do," but its meaning is about the same as "Hello," so you can use it without a name any time you could say "hello" without a name. It's informal, so don't use it in formal situations.
If I were you, I would avoid using it unless you find yourself living in a part of the US where the locals say it all the time. (I have never lived in such a place and so have never had it as part of my everyday vocabulary.)