What about a gold locket with her wedding date engraved and a two little pictures inside.... one of her and one of her fiance.
If your looking for something a little less expensive and you have a creative side, it is always nice to put together little videos with Windows Movie Maker or something. You can compile a collection of pictures and video clips of the happy couple with their wedding song, or any beautiful love song playing. If you can get together the photographs, I bet she would love a scrapbook with all pictures of her and her man as babies, kids, teens, pictures of them together and what not. It is alot of work, but if you have the time and the pictures it is alot of fun and your niece will cherish it.