I once kept the newly unique to me empty 35mm cassettes, and would tear the ends off and squash them and then stick them along a moulding rail that ran near the ceiling of my old darkroom. Now that the variety of films I find/are offered any more is shrinking I keep that collection in a drawer.
Similary I keep unique to me 120 backing paper sections taped to spare old quad paper pad backing cardboard. I keep then in a filing cabinet, and look at them once in a while.
I keep any non-crimped 35mm cassettes for reloading. Some are over 25 years old now, and still work well. Other crimped ones with an inch or so of spool end tail hanging out for reloading I now keep to reload as weel, since finding a nice leader retriever at Freestyle a few years ago. I keep the empty cassette box limited to about 50 units, with the film canisters to match.
Spare canisters I give to the scout groups in our area from time to time. They use them for storing spices for thier field mess kitchen kits.
I keep about 20 respooled 120 backing paper and spools. I do reload from slit down bulk 70mm from time to time.
Five or six empty rolls of 120 backing paper folded back on itself and some masking tape and a staple gun have been used to light proof a window frame or door, and still alow the window or door to be operated in some imprompu darkrooms that I have set up in the past.