One of the best non-registry, non-$$ wedding gifts we got was 2 friends took our wedding invite and had it framed and matted for us- and they had an artist paint flowers on the border of the invite as well. It's really lovely- I just wish they had looked at our registry for hints in terms of colors we like. The matte is plum colored and we are more red/khaki/green people.
If you are going to go off of their registry I would get them something a little more "special" maybe something w/ their new joint monogram on it? We got a great engraved vase that was from Tiffany, but that would probably be out of your price range. What about a nice picture frame w/ their monogram and/or wedding date on it?
Having the wedding invite framed is really nice... I am so glad we got that, because otherwise it's not the type of thing I would ever get around to doing myself.