Envelopes. This obviously works better with the long envelopes that arrive in the post. "Windowed" envelopes won't work (those with a cellophane window to display the name and address inside.) I use this method on invitation cards and thick envelopes, too, but be prepared for the occasional tearing and wastage.
A sharp paper-knife, of the sort I have photographed. The blade needs to be thin, long and sharp.
Slit open the glued folds of the envelope, on the width (after having opened it carefully to preserve the closing flap).
Slit open the glued side along the length on the middle of the back of the envelope. Please do this carefully.
Fold the envelope inside out (the flaps on the length should also be reversed, so that the "gummed" side doesn't show.)
Glue the flaps carefully, and let dry. Voila, a nice recycled envelope is ready.
I mark them with my "recycled" trade-mark, a tiny one at the corner of the back of the envelope:
There! It takes about...3 minutes to do, that's all. Particularly with the thick brown-paper envelopes, it works really great...but if you have a sharp paper-knife, you can recycle all sorts of envelopes.