Have you considered driving back from Fox to Christchurch instead of taking the train both ways? It would give you a different view of Arthur's Pass and be less redundant than taking the train both ways. It'd also save you having to collect yet another rental when you got to Christchurch, saving time, unless you're planning to take a tour or bus.

Have you done the math on these separate car rentals?

Or have you considered not going to the west coast glaciers at all? Especially if you don't like to drive. You can see glaciers inside Mt. Cook park, if you drive between the Queenstown region and Christchurch.

The Hooker Valley walk is interesting.

If you do decide to drive between Queenstown and Christchurch I think the route is much more scenic. There is quite a lot of lodging in Methven since it's a ski resort in season. One of my best lodging luck-outs ever, got an entire house to myself for a few nights for a single-bed rate of $20 a night. I just happened to be driving by and the two ladies that managed it were out front. I promised not to mess up anything other than my own bed.

There are some cute little cottages in Arthur's Pass that can be rented. It used to be that they would split up the cottages and just rent out the bedrooms, but now I think you have to rent the entire cottage.

http://www.trampers.co.nz/

I notice that there is a word missing in the above posting.

The inland route is more scenic bet. Q'town and Chch. It's the one that gets labled Windwhistle or Rakaia Gorge scenic highway.
A combination of 77 and 79.

If you run out of time the main highway 1 connecting near Geraldine is faster, though.

<It used to be that they would split up the cottages and just rent out the bedrooms, but now I think you have to rent the entire cottage.>

They only rent the entire cottages during the low season mlgb, otherwise it's on a room-by-room basis.