Prince Caspian was always going to be a problematic book to adapt. It is, as you say, Steve, a plotless mess.
I remember, when the film came out, that someone complained here on TrekBBS, that the film felt like a reboot of the franchise. "It's the second movie in the series," the poster complained. "Why reboot?" But that's what the book does. Everything we knew from the first film is gone, because Lewis kicks the story ahead two thousand years or so. (The "lost" Narnia appears again, in A Horse and His Boy, but I don't expect to ever see that made into a film.) The "main" Narnia storyline begins here, in Caspian, and the important characters and arcs debut here. LWW is, when viewed dispassionately, a prologue of sorts.
The screenwriters grappled well with the problems the book poses. On the one hand, Lewis doesn't deal with the ramifications of four children growing to adulthood, then reverting back to children, and what this would do to them. On the other hand, Caspian isn't a story about the Pevensies; they arrive in Narnia, get told the important story, and show up at the end and defeat Miraz. Anything the screenwriters did was going to be an improvement over Lewis' structure and story. We can quibble at the need to ramp up the action, like the attack on Miraz's fortress (which, as it's presented in the film, makes absolutely no sense; how did Peter get his army across the water in the first place?) or the Lord of the Rings-esque final battle. Yet, they're not bad creative decisions; Lewis just didn't give them a lot to work with in the book.
Well, maybe the bacchanal. But that's not something Walden Media would have wanted to touch with a bargepole...
Dawn Treader, if it's made, could be fantastic. It's a strong book, it's an adventurous book.
I would probably stop the film series there.
