I'd say definitely the timeline in PC, while I miss seeing the Nurse, had they kept it as it was in the book... the movie would have been a huge mess... I found the size of the flashback to be rather boring in PC actually...

Gael was another change I liked, though I wished to have seen more of her... it was interesting that Rhince(?) had a family. I also liked the Fox in LWW, he was a very well thought out character. And he played a rather significant part in the story, which I think bettered it in the end.

One of the best things they did in the movies was really flesh out Miraz character. Also the Bombing Of London was nice.

I'm going to focus on changes rather than additions.

~I actually liked the changes they made in LWW as to how the children get into Narnia. Particularly with Lucy discovering Narnia for the first time during a game of hide and seek.
~I liked the scene where Edmund meets Tumnus in the White Witch's prison. That's certainly not what happened in the book, but it worked very well in the movie.
~I liked the entrance into Narnia in PC in the train station. That was pretty cool.
~I liked the way the timeline in PC was rearranged. I knew going into the movie that they were going to have issues based on the way the book was written, and I do think that the movie could have done it better, but honestly, I thought the timeline turned out pretty good.
~While there were many parts of this scene that I did NOT like at all, I did like the idea behind the attack on Miraz's castle. I think that with a little tweaking that could have been one of the best changes made in all of the movies.
~And last but not least, I liked the slightly changed scene of the entrance into Narnia in VDT. It wasn't a big change, but I thought it was a good one.

I liked the idea of the castle raid, though I felt like they could have executed it much better.

In the book, we know that the Pevensies dropped in on Caspian right after a battle with horrible losses, but we don't actually see that battle. All we see is the aftereffects, of Caspian's devastation and desperation. We don't actually see why Nikabrik was so desperate he turned to black magic. You could argue it doesn't matter, and it doesn't really. There is no excuse for evil.
But I still felt that seeing the battle was an excellent way to show through a movie medium just what our little band of rebels is feeling.

I'm sure I'll think of more changes I liked... Well, maybe.

I think the best change in the films by far was, ironically enough, in VDT. And no, I'm not referring to character changes or green mist. I am referring to the Journey Back. In the book, Aslan returns the Pevensies and Eustace through "The Door in the Sky", which while perfectly fine for a book, is a bit of a puzzle to adapt onto screen. The way the filmmakers handled it--by going from "a rending of the blue wall and a terrible white light" to an ocean wave, and then Lucy's bedroom slowly draining out, was nothing short of brilliant, stylistically speaking. Such a great way to end the film, no matter how you might have felt about the rest of it.

Another change (although it's less of a change than it is an expansion) was the bombing scene at the beginning of LWW--a great example of Showing vs. Telling, as another member noted elsewhere on the forum. Other deviations such as the cricket ball (heehee) and the ice river were quite acceptable.

The Castle Raid in PC is arguably the best planned attack in the Narnia films, if only character flaws hadn't gotten in the way. I think if it'd been executed better, that could have been an extremely powerful scene. As it stands, it's still not half bad.

I liked the intro to LWW, where one or two sentences in the book were expanded (perhaps not really changed) into several scenes, which made the situation at the time of WW2 clearer to a viewer of today - to a reader five years after WW2 ended, it could be taken more as a given that they would know what it was all about.

I also liked the addition of a scene with Edmund and Tumnus in the Witch's dungeons. We get to know Tumnus better, and Edmund begins to see what he has done.

And the final battle in LWW - in the book we only see the last minute of it, and one small but important moment is told in flashback - for a movie it makes a lot more sense to play it out more fully, like it was done here.

The added Night Raid in Prince Caspian was a good change. It helped advance the character development and theme of the movie.

I also loved the Entering Narnia scene in Voyage of the Dawn Treader. I think if it had been done the way Lewis described it, it would have been very cheesy. Same thing with the Return Back Home.

I liked Edmund sitting on the Witch's throne, and dungeon scene. Two examples of the filmmakers taking scenes from the book where we could hear the characters' thoughts, and finding ways to ways to convey them visually.

I also loved Lucy's dream in PC. It solved the structural problem (it's too early for Aslan to return), and visually conveyed the sadness of the old days being gone. We see Narnia as it once was (colorful, full of life) in Lucy's dream, and then in the next scene we see how it is now (dead). Solid visual story-telling.

I think a change that I really liked was in PC at the train station when Susan said, "We live here now. There's no use pretending any different." It was a subtle line, but fans of the book that know the future of Susan appreciated the foreshadowing of her fate. It will make it easier and more believable for her to no longer be a Friend of Narnia if LB ever gets made.
I also agree with others about the London Bombing. Overall, just little things were appreciated.