Collectible Charms for Scales?

The basic look of the charms I can create

I've been working on a new incentive for students who master certain scales - collectible charms. I create them using a groovy charm maker I bought last year. So what kinds of charms should I create? Composers? Cool-looking pianos? General musical?

And how should they be earned?

Should they cost any money?

And what about boys? Are there any manly ways to use charms?

Here's what I'm thinking right now:

• Each charm would have to be earned with demonstrated mastery (memorization, correct fingering) of something like three scales.
• I don't know if I should separate the scales into groups myself or if I should allow students to select which three they want to do. If any piano teachers are reading this, I'd love to hear your thoughts. It seems like it would be easier to keep track of who has done what if each charm has a corresponding set of three specific scales. That way, students will be encouraged to master some of the tougher scales in order to earn certain special charms.
• I can hear future conversations between students now... "Oh, you have the Carnegie Hall charm! What scales did you have to do for that?" At least I hope to hear things like that.
• If you play these scales one octave, the back of the charm will be blank/empty. If you play them multiple octaves, there would be some sort of extra image on the back of the charm.
• Each charm would cost 50 cents. That would cover the cost of making them (I think), and it's affordable for students.
• Students could choose to wear or display the charms any way they like - on a chain, a bracelet, a hook of some kind, etc... Do you have any additional ideas about this? I'm particularly thinking about the boys and wanting them to dig the idea.

Possible charm ideas:

• Bach (C Major, G Major, and D Major) + 2 or more octaves ("Epic" on back)
• Beethoven (A Major, E Major, and B Major) + 2 or more octaves (Josef Hofmann on back)
• Brahms (F Major, B-flat Major, and E-flat Major) + 2 or more octaves (Diamond on back)
• Bartok (A-flat Major, D-flat Major, G-flat Major) + 2 or more octaves ("Beast" on back) 
• Grand Staff (A Minor, E Minor, B Minor) + 2 or more octaves (Chopin on back)
• Organ (F-sharp Minor, C-sharp Minor, G-sharp Minor) + 2 or more octaves ("Like" on back)
• Harpsichord (D Minor, G Minor, C Minor) + 2 or more octaves ("Ear Candy" on back)
• Grand Piano (F Minor, B-flat Minor, E-flat Minor) + 2 or more octaves (Carnegie Hall on back)

So, anyway, this is what I'm brainstorming. Got any better ideas? Tell me!