I and my family are kite fliers, we get through a lot of outdoor style zip off trousers, somehow the cheap ones only seem to last a couple of years so there's a ready supply of them between us.
I'm also a recycler, I hate to see anything go to waste, so I present some trouser bags:
The netty ones we use for the veggies we buy at the farmers market, they're the linings from the knee sections, finished off with long scraps (you might spot a waistband and the neck from a teeshirt).
The solid ones are made from the ankle sections, these ones happen to be lined (unusually these were lined all the way down) with a satin-y sort of material, they get used for all sorts of things: line bags, peg holders, waterproof stuff sack e.t.c. I don't make these with a use in mind, but I never seem to have enough of them either. They are so easy to make.
Recipe (for the more common, unlined variety):
Take a pair of double zip off trousers, salvage all the zips from them for other projects with a seam ripper.
Take an ankle or knee section.
Pick an end to be open (nearly always the base for the ankle section, it matters a lot less for the knee section).
Turn inside out and sew across the bottom.
If using an ankle section, turn right side out, thread some new elastic or cord around the top (or not if you've been really lucky and the trousers haven't gone in that section-ours always have though).
If using a knee section, sew around the edge (1-2cm from the top) from seam round to (the same) seam leaving the last 1/2 inch either side of the seam open. Turn right side out, thread some new elastic or cord around the top.
My favourite part of this project is that the trousers have had the seams sorted out already, no need for ironing.