Some people collect shot glasses, t-shirts, hats, pins, postcards, you name it! I've spent a lot of time traveling with the bug since she was tiny and I wanted to find something that we could collect that she would actually enjoy using while also learning about the places we've visited. Our home is super small so it definitely couldn't be something that took up a lot of space.
Hence the book idea.
Sometimes it's not the easiest task but we've always found something that has related to where we've gone! We usually have the best luck in quaint little gift shops that pop up along the way.