Baseballs, for the most part. I also order art prints of the MLB ballparks I visit from a website online. Can’t recall the name offhand but I have the link saved somewhere. Oh, and I save tickets and programs whenever possible as well.
I can’t blame you for not wanting to bring a Yankees hat into your house! I made the mistake of not collecting anything as I went to different parks. I’ll need to change that on my next lap, and will be watching this thread for good ideas.