First, for the kids in the "middle readers" category (generally, kids aged 8 to 12)...some of these are just as easy as the Ramona books, but others are a little more challenging. I am including the easier ones because, even though JP is 13, they are books he still enjoys rereading when he wants something simple and fun. Let your kids read both below, at, and above their level...reading below their level on occasion gives them the confidence to tackle harder material.
Grace Lin is both a fantastic writer, and an amazing illustrator. There are several links on her website for activities, more info on her books, and more. There are also some interesting videos. Although she has more than three books out, these are the ones we have read:
If you aren't familiar with the Wayside School books by Louis Sachar, these are very easy to read, and loads of fun. We've read many of Sachar's other books, and have several in our home library, but these are the ones JP has enjoyed the most.
For more hilarity, check out Carl Hiaasen's books for younger readers. Although not pictured here, we have also read his book, Scat.
For a child with an interest in World War II and Holocaust history, Art Spiegelman's Maus and Maus II are fantastic. They are graphic novels, and mice depict the human characters which I think greatly helps younger readers to synthesize the horrible atrocities of the Holocaust. I couldn't find a website for Spiegelman. If you know of one, please let me know. Thanks.
Amended to add....I don't have them with me as we still have many boxes at my mom's house in California, but I wanted to add a few more books to my suggestions for middle readers. These are thicker, and a bit more challenging, but still middle reader appropriate:
Anything by Cornelia Funke, but especially The Thief Lord, and the
Inkheart trilogy (Inkheart, Inkspell, and Inkdeath).
Skinnybones by Barbara Park - This was a favorite of mine when I was in
grade school, and my son enjoyed it too.
Any children's book by Bill Wallace, but especially A Dog Called Kitty. Many of Wallace's children's books focus on the relationship between children and animals, in particular dogs.
And, although it has been made into a movie, I highly recommend the book How to Eat Fried Worms by Thomas Rockwell.