Do you want to focus on telling kids to bring in the tops or you want to make it seem fun and exciting by showing who's brought in the most?
Because you could put a mountain and Reaching the top with Boxtops. Each grade rather than teacher could have a path to the top of the mountain with a climber. THis would promote a cooperation within the grade levels and encourage students to work together as a grade.
You could do the same with whatever theme you like or could find at the teacher store or make if you are creative. I would also speak to the art teacher, they always seem to have great ideas and it would get others involved in the school.