There are so many things that can be used as swag to market your book. Some are highly suggested for every authors. Others can be dependent on the subject of your book.
Bookmarks/Postcards - Every author should have bookmarks made featuring their work. Readers use bookmarks over and over again. Every time they open their book, they see it. That is marketing that occurs over and over without you having to lift a finger. They are cheap to make and easy to distribute. Postcards are used by many authors. I see them as oversized bookmarks and can be useful promoted that way.
Pens/Notebooks - You can have these printed with logos and the name of your book. I can always use another pen or notebook to jot down ideas. You can never go wrong with this kind of swag.
Jewelry - This can catch the eye of the reader real quick. Is your story about a werewolf? There are necklaces, earrings, and pins with wolves on them. Maybe pearls are featured in the story. Give away pearl necklaces. They don’t have to be expensive. But a few given away goes a long way in marketing.
Specialized Items - These are ones that relate specifically to your book. A great example is a romance writer I know who wrote a book about two chefs falling in love. She gave away aprons with the name of the book stamped on them. What a cool idea! What is unique to your book that you can give away as swag?
You use swag a lot when a new book is coming out or you are celebrating a special event. You might give it away as prizes, but they are mainly just gifts you give out at live events, as a thank you to reviewers, and to leave in public places to promote your work.