IMO, an engagement ring is rather quaint, but a very poor investment. The mark-up on jewelry, especially gemstones, is astronomical. If you've done a prenup, you obviously have your head screwed on the right way (and are keeping the hormones under control).
I would suggest a washing machine. This shows that you're serious about her moving in and gives years of useful service. Particularly useful when the (non-disposable) diapers will need taking care of.
<Did I say IMO?>
<BTW, this is one of my more serious posts.>