In the world of the Witcher, when someone saves the life of another, they can invoke the Law of Surprise to receive that person's next surprise. In the show, the only example we see of the Law of Surprise is a child of surprise (for both Ciri and her mother). However, it is implied that a child of surprise is very rare.
What are some other examples (from books, games, etc.) of what can be given by the Law of Surprise? Does it have to be something substantial or can it be anything?