Great question. In my experience the host families "adopt" you for the summer and in a sense, treat you as if you're their own son. In return, I've gathered that the majority would just be extremely happy if you were to keep in touch with them. However, a gift would be great as well. My best suggestion would be instead of just a one-time gift, to make it constant small things. That way they feel as though they impacted your lives in a way that you appreciate enough to keep them up to date with what you're doing (because after all, they did). I've done school gear, pictures, copies of yearbooks, etc. I plan on taking my host family brother (10 years old) from the past to a Nationals game when time constraints allow, being that he lives about 80 miles west of DC and I live in DC. Often times it is the thought behind the gift that is more important than the actual tangible item.