Bracelet over bangle? I am confused. Are you talking about the Cartier love bangle versus the Cartier love cuff or the diamonds by the yard bracelet? Maybe you should go with your first initial gut thought, the right hand ring. Or earrings. Both of these are easier to buy. My husband gave me a pair of pearl studs the morning of our wedding (well, technically it was my sister as we followed the whole superstitious thing about not seeing the bride before the wedding) and I was so touched. They are very classic and I wear them for special occasions. Pearls are yet another idea and I have bought from americanpearl.com and been extremely pleased with their quality. However, the are very traditional and you would need to know if she would look good with a rose or silver overtone (It depends on her sikn color).
Did anyone suggest the silver amethyst sugar stack ring:
http://www.tiffany.com/Shopping/Ite...+1-p+4-c+-r+-x+-n+6-ri+-ni+0-t+sugar&search=1
or the amethyst sparkler ring (It comes in large too.):
http://www.tiffany.com/Shopping/Ite...p+4-c+-r+-x+-n+6-ri+-ni+0-t+amethyst&search=1
Check out
http://www.gemvara.com
It is a customizable fine jewelry site. They will make any of their pieces in amethyst, diamond, silver, white gold or platinum and they show what it will look like as well as what the new price would be with your changes.
Another idea is a charm bracelet that you can personalize with charms that have a special meaning for both of you and she can add to later, however I do not think that that is something for her to wear on her wedding day. I would recommend Tiffany's or for a more personalized look, Heather Moore:
http://heathermoorejewelry.com/products/Charm-Bracelets
Something more everyday might be diamond stud earrings if she does not have those yet. I have the DBTY ones and wear them every day. They are definitely a staple and many ladies on this forum have a few pairs in different size and/or cuts.
Just an FYI, I am not affiliated with any jewelry sites that I posted. Just giving futurehusband some more ideas.