Most Helpful Girl
I don't know I think it's a cute idea. It shows her your intentions even though you can't get her an engagement ring. I would like to get one lol. You don't have to tell her it's a "promise ring" you can just say it's a pre-engagement ring or something
Most Helpful Guy
Ehh, seems kind of childish. I would suggest a locket or some some other form of jewelry. Of course, me suggesting things takes all the romance out of them. Figuring it out for yourself will be all the better.