I never say "Hi/hey there." Maybe if I forgot someone's name this might fit the situation. Just say Hi or Hi + NAME.

Hanna, "slightly" more formal? Like, I mean, how much more formal than "hello" can you get? =:^) (Oops, I forgot "how do you do?" Now just when was the last you used that old anachronism, anyway?)

Use of hi or hey depends on region, I think. In the South where I am we say "hey". There was a special Monet show at the museum here, and the advertising told you to go and "Say hey to Monet."