There are four or five primary characters in The Wednesday Wars by Gary Schmidt, and each of them is distinctive. Here are a few ideas for symbolic items which might represent each of them.
Holling Hoodhood is the protagonist of the story, and some fitting objects to symbolize him might include cream puffs (which keep coming up in his story), the two rats (Sycorax and Caliban) which he inadvertently unleashes on the world, or a complete works of Shakespeare, which of course he enjoys reading. The most interesting visual image, though, is his Ariel costume. Holling says,
[Mr. Goldman] handed me a pair of bright yellow tights with white feathers on the...well, I'll let you guess what part the feathers were attached to.
Mrs. Baker's items might also include a book of Shakespeare's plays, but she could also be symbolized by the wooden box holding her Olympic silver medal or a telegram. The medal is representative of her life outside of teaching, and the telegram is indicative of her husband who is off fighting the war in Vietnam and becomes missing in action.
Heather Hoodhood might best be represented by some kind of flower-power symbol, such as the one she paints onto her cheek to demonstrate her connection to the peace movement. She also leaves home in a Volkswagon beetle, which is another representative item of her protest behavior.
Mr. Hoodhood would be best represented by a business card with his company name on it: Hoodhood and Associates. Since his major concern in life is business, this is an apt item to depict him. Another choice might be some kind of an award certificate as businessman of the year as presented by the Chamber of Commerce, as this is represents the pinnacle of success for him.
Each of these characters in the novel is distinctive, and these are the items that might best symbolize their personalities as well as their interests.