Our favorite spot on the South Island was Edoras - filmed on Mt. Sunday. It's a long way down a dirt road, and we had the place to ourselves. Terry had fun finding anchors in rocks that were used when the hilltop village was built for the movie. The hike itself was quite easy. We are so glad we didn't skip this one, which we considered doing when we had a wreck the day before and were a little behind schedule.
The fictional location of Isengard is near Paradise, NZ. It's about an hour's drive from Queenstown (a very scenic drive!). There isn't an exact spot shown, and we know that in some cases mirror image shots were used, so we didn't want to use too much of a beautiful day trying to pin down an exact location. Here's the general area.
And we also saw the filming site for the Battle of Pelennor Fields. Google Maps actually leads you to a spot. We did hear that almost everyone who lives in the town of Twizel was an extra in the movie.
And when we were the very furthest south that we went on our trip (and the furthest south we have ever been in our life), we saw the river where many of the river scenes from Fellowship of the Rings were filmed. We took a long hike near there, and if we're ever back, we'd love to float this river.