Journaling for summer reading?

I have to read three books this summer and journal a page & 1/2 every 20 pages. I just can't seem to think of enough stuff to write. You can't summarize the book, it's more of your opinion, thoughts, and questions on the book. The only thing is, I really have no reactions. I keep unintentionally summarizing. Right now I'm working on The Great Gatsby. Any helpful hints/tips to get me a page and a half?