First of all, congratulations, you are about to meet your parents-in-law. Perform well and I wish you success. For his father's birthday party, you have to dress appropriately and not be too trendy. Although parents nowadays are not so feudal, they are still very conservative. You can wear a small T-shirt with a pair of jeans to let them see that you are a sunny and lovely girl. As for gifts, you can give some gifts that they like at their age, such as the commemorative version of the Golden Years, so that his parents will miss the time they spent together. If his father likes to drink tea, you can give him a living porcelain pomegranate cup. Pomegranate flowers are a symbol of wealth, auspiciousness and prosperity. Living porcelain can also enhance people's immunity. This is a healthy gift.
You can also give some gifts such as collectibles, which will not only make the future father-in-law happy at the time, but also this gift will have room for appreciation in the future. For example, purple cinnabar paintings are good gifts for collection.
No matter what gift you give, it is your heart, and I believe his parents will understand. You have to show your best side and I wish you success.