Although I've never read Alien Secrets, I've heard about it. For your first artifact, try thinking in terms of the conflicts in the book. Something has been stolen - that's one conflict. Puck is in danger - from whom? (character vs. character). Both of the main characters are returning home in shame (character vs. self). What artifact can you use to show one of these conflicts?
The second artifact is about the plot. Since the Soo is stolen, bring in your own version of the Soo.
The third artifact could be something that you relate to the book. For example, since the main character is Robin Goodfellow, who gets her nickname from Midsummer Night's Dream by Shakespeare, bring in a copy of Shakespeare, or something related to Shakespeare.
Since there is a ghost in the story, bring in something Halloweenish.