If I were going to someone’s home, I would bring an item already suggested above and follow up later with a hand-written card.
However, if it were a restaurant, I wouldn’t bring anything. Instead, while at the restaurant, I would try and be pleasant and try to figure out if they like a particular store (based on their conversation, article of clothing, etc.)—or even tactfully express admiration for their shoes/clothing/anything else and find out where they got it.
Then, I would surprise them later with a sincere hand-written thank you card and a gift certificate to a place that they mentioned they liked. As an alternative, a hand-written thank you card and some flowers delivered to them would also be a very nice gesture.
All you really want to convey is that you are/were genuinely interested in them and sincerely appreciated their hospitality. So long as it is genuine and sincere, whatever you do will be just fine. Don’t worry so much about “etiquette” and go with what feels right to you.