there are these very neat bracelets, sterling silver... and I think the bracelets are like $40 or so... and charms are $10-30. So you could do a bracelet and a charm for each. And there are soooo many charms, so you could do the one with their birth stone or something special about your friendship.
I want to get one so when our baby is here I can do a charm on it for memorable things with her.
Oh and the charms don't hang like necklace charms, they are kind of like rings... go see here :
http://www.pandoracharmbracelets.com/