Old people like antiques, such as my grandfather. He doesn't like anything else, but he likes collecting pocket watches and writing brushes. A few years ago, my grandfather celebrated his birthday. I always bought him a very big birthday cake, but I didn't see how happy he was. Later I learned what old things he had been looking at, such as a pocket watch. This year, I bought him a pocket watch, which made him very happy.
We can also search online to see what gifts netizens usually buy for the elderly, imitate them themselves, or directly ask the elderly what he likes. Actually, this is the best way.