Grapes is no medium read--it is woe all the way! I think I would rather stick needles in my eyes, or roll around in poison ivy than take the Joads on vacation with me!
I'd opt for Gatsby, though it is not a tale of great joy, because I think it is a fascinating vignette of life in the Fabulous 1920's for the elite...even though there is tragedy, there is much to think about with Gatsby.
If you want a really good book to read, pick up McCullough's bio of John Adams. It has just been released in trade paperback, due to the release of HBO's mini-series of the book. I read it when it first came out several years ago, and could not put it down.
Happy holiday--whatever you decide to read!