Kinō wa gochisōsama deshita (昨日はごちそうさまでした): thank you for treating me yesterday. A manners guide in Mynavi News advises adding these words to the greeting the next morning after being treated to a meal or drinks: Ohayō gozaimasu. Kinō wa gochisōsama deshita. Good morning. Thank you for yesterday.
A written thank-you, the guide adds, should go out the same day or, at the latest, the next day.