Sirius walked into Cabin 7 hoping not to run into Aubree. After making sure that she wasn't in there he pulled the note out of his pocket and read it 1 more time:
Dear Aubree,
Change of plans on our date. Meet me on the edge of the Forrest tomorrow, wear what ever you'd like. You'd look beautiful in it anyway.
With Love, Sirius
This would be perfect. Tell her just enough to know where to go but not enough to ruin the surprise. He found her bed and left it on there, half open so she wouldn't miss it. Then left the room smiling and whistling to himself.