I am assuming you are interested in prop ideas for some kind of presentation or book report for The Wednesday Wars by Gary Schmidt, items which would be interesting to show but will also connect directly to the story.
The first item that comes to mind is a rat or two, of course (and I would suggest fake ones). Sycorax and Caliban are an ongoing and rather humorous part of the story. They are connected to Mrs. Baker's husband, Mrs. Sidman's sudden and dramatic heroism, the infamous cream puffs, Holling's classroom, and Camillo Junior High School, among other things.
Another item would be yellow tights and/or white feathers. These are the bits from Holling's Ariel costume which we hear about consistently. A variation on this might be a replica of one of the fliers which Doug Swieteck's brother posted all over the school.
A tasty choice might be a cream puff. They show up in both positive and negative ways in the story, and they are connected both to Mrs. Baker's military wives organization as well as to Goldman's Best Bakery.
A Complete Works of William Shakespeare book would be perfect to represent Holling's growing love of Shakespeare throughout the novel.
You will probably want to include something to represent Holling's older sister, Heather, so anything which appears to be from the '60s hippie movement would be suitable. Anything that references flowers or Volkswagon beetles, for example, would be terrific.
I'll add one more your teacher might appreciate: the infamous red pen Mrs. Baker uses. If you use this one, be sure to reference this quote:
“I handed the test in five minutes before the end of the day. Mrs. Baker took it calmly, then reached into her bottom drawer for an enormous red pen with a wide felt tip. "Stand here and we'll see how you've done," she said, which is sort of like a dentist handing you a mirror and saying, "Sit here and watch while I drill a hole in your tooth.”
There are obviously all kinds of other props suitable for this novel, but certainly any of these would work for you.