I agree with the suggestion of a lap quilt. As much as I would appreciate and respect the love that went into a wall hanging, it wouldn't go on the wall in my house--it would just look out of place. Lap quilts, however, will go with pretty much anything, especially if you choose a simple, classic pattern like log cabin or Irish chain. Make it a generous size and they can snuggle under it together. :)
I also agree about maybe waiting until they have a registry somewhere--that will give you a clue as to their color taste and style.