My son just got back from the NECBL last night. In my book his host family were Saints.
We helped our son with a couple ways of saying thank you. His gift to them was a "commerative poster" of his season which he inscribed with a writen personal thank you message. We just completed a thank you card made up of photo's that we took of our son with his host family during our trip back east. It was printed as an oversized postcard which he signed and sent off to them.
We also made a point of taking his entire host family out to a nice dinner when my wife and I came to see him play.