Watney created a room from an emergency pop-out tent on the rover. It was dangerous but he wanted a place to relax and not go mad spending all those days cooped up; he also spent a lot of time travelling and laying out the solar panels on his long trips.
The RTG that Watney uses to generate heat does it so efficiently that he has to strip insulation from the rover to manage his body temperature and not put himself at risk. He then creates a makeshift insulation panel to help with this process.
Many obstacles Watney faced were excluded - some of these could have significantly added to the run-time, including the 'roll-over', drill malfunctions and a resulting communications blackout. He also has to deal with a severe dust-storm on his final journey to the MAV.
When one of the airlocks breaches, the situation is more complex in the novel. Watney's helmet suffers severe damage that he has to repair with a resin, which is so tough it impairs his ability to use a hand in the resulting escape attempt. First, he has to locate and block a hole in the airlock that oxygen is escaping from. Then, he then has to spend a full day slamming into the airlock with his back to cause it to roll, so that he can move closer to the HAB, a vital process due to his diminished oxygen reserves.
The Hermes's return trip to Mars is more expansive in the book. More time is spent on the difficulties the crew face. We learn here that Beck and Johannsen are secretly together, although Lewis knows about it and allows it. In the movie their relationship is hinted at being more recent than that.
Watney never executed the Iron Man idea, although he did suggest it. In the novel, it's Beck who retrieves Watney and not Commander Lewis.
After being rescued, Watney speculates inside the airlock that, were it a movie, the entire crew would have been there to see him rescued. Because they have duties to execute so that the return journey is safe, this doesn't happen. It's likely that Andy Weir told the film-makers to include the entire crew in the film, as a nod to this line of thought.
The book ends shortly after the rescue of Watney; everything after that is elaborated by the film-makers.
Throughout the novel, we are given much more of an insight into Watney's mind due to the first-person narrative perspective in which the Mars sequences are written. This means that more time is spent on his internal monologues, and more is spent on his emotional difficulties in being left behind. At one point, after discovering he can communicate with NASA, Watney breaks down and weeps for some time, simply relieved to be able to speak to someone. Although we do get a glimpse of this raw emotion when the airlock dismantles in the film and when he's about to be rescued by the crew, it is much more expansive in the novel.