Printable coloring book pages — we've talked about them here before. They're popular because kids prefer coloring pictures of characters and scenes they already know and love.
For better or worse, my kids love the Captain Underpants series by Dav Pilkey. At first I found the potty jokes and prankster glamour irritating, but having a seven year-old boy requires opening one's mind to new forms of humor, and I'm now a big Pilkey fan myself.
Pilkey's illustrations are bold and cartoonish, perfect for coloring. But, like many, I have a built-in revulsion to my kids scribbling in their "reading" books. So, I photocopied a few two-page spreads using our HP All-In-One printer (printer/fax/copier…the greatest piece of hardware in the world), and now my kids can scribble to their heart's content on pictures they really care about. Bonus: it helps their reading skills at the same time.
The ink usage is high, so if you find yourself doing this often, it's worth a trip to Kinko's or similar photocopy shop.