I agree with Sara that consumable gifts are great. I usually bring something consumable and kind of luxurious, like fancy chocolates or wine, that can be shared. I often go to a fancy local chocolate shop in my town so it's something a little more unique than a box of Godivas. Although Godivas are great too and I would love to receive them. Or fancy tea if for some reason they are not chocolate/wine types... but really, who does not like chocolate? (Although I suppose someone did once tell me NOT to bring chocolate b/c someone in the house was on a diet...)
Another idea that I think is very thoughtful (assuming you and your hosts are amenable to the idea of you using their kitchen) is to cook them a nice meal one of the nights you are there.
ETA: I think a post-stay gift is a good idea too, but I would think of it more as a thank you gift than a host gift...
