A few things in the north that come to mind include John's election to head of the Knight's Watch and his gaining Stannis's respect when he beheads the guy who used to be commander of the Gold Cloaks (forget his name, sorry). I would think we will also see the Onion Knight sent off to try and find Rickon (there's some different stuff involved in that in the books including him supposedly dying even though he really doesn't) - he goes to see Lord Manderly who is secretly doing some things to undermine Bolton. There's also a pretty sweet scene in the books where he feeds an entire wedding party special pies (the North remembers!). They may even take it to Bolton's winning over the hill tribes of the north and marching to the ruins of Winterfell to face off with Bolton. I'm sure we'll see more of Bran and just what the faces in the tree are and who the old gods are (including his "visit" to Theon). The season might end on the Jon/Stannis cliffhanger that the last book ended on.

We'll see more of Dorne I suppose and Danny's troubles maintaining order in the city she's in. They might even wrap the heir to Dorne visiting her to try and enforce the deal her father made with the mad king back in the day to marry her to the prince of Dorne. They could probably cut that entire scene if they want to since it really went nowhere in the books (it doesn't work out well for the prince in the end).

Since they want to finish this thing in 7 seasons they're also going to have to move Tyrion's storyline forward somewhat quickly including his "capture" overseas and his captor taking him toward Danny. It will be interesting to see if they include his sailing with the mercenary company and if the young man in the mercenary company is included since it will answer a big question in the book as to whether a young boy at the end of Robert's war really died or not.

There's a lot of stuff in the books in King's Landing too between Cersei and Margaery including both of them getting arrested by a resurgent priestly order (I forget their names, but Cersei lifts their ban to help pay off debts and then they go about basically like a militant holiness movement enforcing a lot of religious stuff). This is when we are introduced to Cersei's "champion" which is clearly Gregor Clegane Frankensteinized by Qyburn.

Oh, and of course we're going to see Arya join the Faceless Men and train to become an assassin.

I'm sure others can fill in some more blanks for you, I frankly have forgotten quite a bit. Since spoilers don't bother you, you can read a summary of the books in Wikipedia or chapter summaries on the GoT forums/website.