Send them a nice card... inside write something like:
"Dear Mr. and Mrs. Blah (whatever their last name is)

Thank you for your hospitality while I was staying at your house.

Sincerely,
FULL NAME
2006"

Just make everything formal... like a business letter. I think that's good. Even better: print it out from a printer, use nice paper, but avoid handwriting them. I did that to my ex-friends' wedding card (I wasn't invited). And no cookies or flowers.