Write analytical papers on what you read. Include the time period in which the author wrote the material, the social context in which it was written, what messages the author wanted to send to readers, and what you thought the material's strengths and weaknesses were. This helps your critical thinking skills and lets you apply what you learned from the material to your own life.
Choose books you normally wouldn't read. For example, if you usually read music magazines, substitute books on colonial American history. If you're not too knowledgeable in that area, you can get magazine articles that will help you understand this era better. The goal is to become well-rounded through books.
Find a topic you've always been interested in and get books based on that. You may enjoy learning about French culinary traditions but you don't have a lot of expertise. Go to your public library or local bookstore and ask someone for suggestions.
Write a short story based on some of the books you've read. The goal is not to get published but to use freestyle writing as a way to challenge yourself intellectually. If you just finished reading a book about socialist movements in Europe, you can write a fictional tale based on those events.
Do crossword puzzles. You can find these in your newspaper or buy crossword books at the local bookstore. It challenges you intellectually and you learn new things through the answers you find.