21 is still young, and I think a necklace would be very sweet.
But if you prefer bracelets and rings, a lot can be done there also. You know, they have Michael charm bracelets which are really cute. You can wear them as is, or you can take the charms off and wear one or two at a time, switch them up, on either a bracelet or on a necklace.
Here's one cute one~
http://www.brooksandbentley.co.uk/product/The-MJ-Charm-Bracelet
Also with rings you could get one inscribed with your favorite lyric or message. That would be really sweet.