What would be a good birthday gift for a friend?

I'm trying to buy a birthday present for a friend this month, but I don't know what to get!

I'm close with this friend because we're easygoing and sometimes prickly, but since we spend the least time together in our group, the only thing I know is their favorite idol group. I'm worried that if I buy them something and they stop being a fan, it won't be useful, so please recommend a gift that is practical but not too expensive for a student!!