How about something environmental...
nature lovers kind of thing...
favorite animal day, wear a recyclable day, bird watchers feather day, nature necklace day, etc.

How about Willy Wonka related;
What are all of the things in the movie?
the last day would be the "golden Ticket"
(there is now a real "willy wonka bar")
maybe wear something blue for the scene where the girl turned into a blueberry, maybe you can sceme up something from that.

Our 4th/5th Grade campus has been doing a huge chocolate candy bar FR for 30 years and it brings in lots of money for our school. We have always made it a HUGE deal and reward the kids daily with daily incentives and at the end of the event we give away four big prizes to the top four 4th graders and the top four 5th Graders. Last year we had a theme which we called "Love America" and we had a different theme every day like love your school day, love your favorite super-star day, love being a kid day, love your country day, etc.
The kids really enjoyed it. Now we are trying to think of another theme that we could do for the whole week that 9-12 year olds would enjoy. Any ideas much appreciated!
Thanks!