Just recently my cousin was proposed to. The guy brough in a huge box in gift wrapping for her. She opened it, and balloons floated out, tied at the bottom was the little box with the ring in it. He made sure it was opened indoors, of course, and had invited his and her family "for dinner".
Just don't get too complicated, or drag strangers into it too much. I know a guy who wanted to surprise his girl at a sporting event with a food vendor being in on it. Unfortunately, the ring was accidentally given to the wrong guy ti deliver. Things eventually got straightened out, but the surprise was most definitely ruined.