[honeychile]

I'm not sure that I could add anything to this excellent list! Quickly:

-Money is out; it just looks crass and/or awkward.
-Gift cards are wonderful. He's going to see up close what they need, so he'll know where to get them. If he's hopeless, ask for their favorite restaurant(s). Sometimes, a gift of 2-3 cards to family restaurants are even more welcome than that "someplace special" restaurant.
-Flowers are good, so long as he doesn't "one up" the husband.
-Candy is more unisex.
-I highly agree with MysticCat; tell your husband to find out when there's a good time to let them alone. He doesn't want to do it on a night when a nice dinner is being made (this happened to me once!).
-And if you need to type it out for him, give your husband some rules on being a good houseguest: hanging up towels, making up the futon, helping with dishes, and generally not being a problem.

Good luck!