Something that makes you think of the stereotypical scene kid. I.e., bows in the hair, hello kitty, etc.
When something can be considered of scene derivation.
Girl: *sees bow in friends hair* Oh my God, that's so scenetastic!
Of, or pertaining to scene.
Something that is extraordinarily scene.
(sees someone with a bow in their oh-so scene hair.) OMG THATS SO SCENETASTIC.