First of all, I must find out what kind of hobbies my girlfriend's father has. For example, he likes to play chess and go, do whatever he likes, buy him a good chess or go, or like to keep fit. For example, if he likes massage or foot bath, you can buy him a massager and a foot bath bucket. If his father-in-law is addicted to alcohol, consider buying him some good wine.
I remember a few years ago, when I was young, my father always took me to visit my grandfather. At that time, my father often brought wine to grandpa, because he knew that grandpa would have a meal every day, and grandpa usually drank some loose wine or medicated wine made by himself. My father will buy him some better wine so that he can drink better wine during the holidays.
Another time, my cousin went to visit his girlfriend's father. At that time, they were not married. It's his first time to see the old man, and he doesn't know what to send. But he heard that his girlfriend's father especially likes playing chess, so he bought a good pair of wooden chess for the old man, and at the same time killed several sets with the old man there, which enhanced the old man's feelings for his cousin. I think this is a very good choice.