Some picture puzzles will work too, Brett. A cartoon which when examined carefully is the name of a book or character. I will send you some sample. I have one in front of me:
A Green van with a "Keep Off" sign on the rear. The side sign reads Gable's Van Service with a tree blocking the first letters (able's ans). Anne of Green Gables is the solution.

A humorous "If XYZ married XYZ their children would be named..." using characters and authors from the books.

Treasure hunt. I have done this one with my garden group.
20 questions. On the answer sheet, write the correct answers. The first letter of each answer spells out where the treasure is hidden. The treasure can be something as simple as a candy bar or a chest filled with bags of goodies for all to share. It is an exam for fun. Or do one per week and have wrapped boxes in the front of the room. Winner chooses a surprise.

Make sure you post the winner's answer sheet on the wall.

The key is to keep it all fun, accessible to all, and cheap.