The $8 bracelets and $3 charms are lots of fun- I've seen them at Mousegear in Epcot, at the shops at AKL and WL, and lots of other locations. These are "dangly" charms- they use little lobster clasps, so they're easy to snap on and off.
There are lots of "character" charms- the fab four, Tinkerbelle, the Princesses, Dopey, Pluto. Also both enamel and rhinestone letters. Lots of Mickey heads- patriotic, sparkly, enameled. Special interest symbols: bicycles, megaphones, sunglasses, palm trees, shoes, musical symbols, hearts, castles... just tons of different charms.
I think choosing charms at each park would be a fun thing for your girls to do, and not very expensive.
(Now, the sterling and gold bracelets and charms are very nice, too. Just not quite as affordable!

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