A simple sterling silver braclet and 2-3 charms will set you back about £200 depending on the charms you choose, which makes your wife a very lucky lady if that's just a "stocking filler".
If you go to a local jeweller that stocks them you will get a better idea of what they look and feel like than just looking on t'interweb. A sterling silver bracelet should be fine unless she especially likes gold, and you can pick out combination silver/gold charms. Pick out a charm that symbolises something, or just pick out letter charms with her initials and your initials.
The idea is that she collects charms from different people or different occasions over time, so don't go overboard just to fill it up. By the time it is filled with charms it will be worth a couple of grand, so it might pay to get it insured.
Hope that helps. I love mine.