Well... some of the things are really tacky, but I think it's best to stick with something simple. For example, my ex and I had matching necklaces with small, sort of unisex star charms. The stars were sort of an inside thing we had, so if there's something like that that you and your boyfriend share, or something you both enjoy, etc, try finding something subtle but related to whatever that is.
Try searching ebay, there's a lot of nice, simple/unisex and inexpensive jewelry on there, or maybe even a bead/charm/diy store if you have one locally and put together your own necklace.
Sorry, I worded that a little strangely, but you should be able to get the idea
