I Have a Question

So, my boyfriend's birthday is around the corner and we'll be going to California around that time, but I don't know what to really get him. Something he'll really like.

Before we leave, I want to throw him a surprise birthday party with his close friends. But as far as gifts, I'm not really sure. I know his hobbies and interests, but I don't know much about them.
He's really into the occult and wanted a book he saw online but I don't remember the name. All I know is it's really expensive. And he's also into card games (he has Exploding Kittens and I don't know if to get him Cards Against Humanity) and card tricks (silly, I know but it's cute). He saw a hat he liked and I'm planning on ordering as soon as I get my paycheck, but other than that, I have no clue on what to get him. A perfume? Clothes? I don't know.