I use plastic milk cartons to store home made seaweed feed. Reuse old labels by soaking them in bleach for a few days, takes all the marker pen off them. Old panes of cracked glass from the GH get cut down to uniform sizes and are used as cloches.The large fizzy pop bottles are cut in half as cloches.
An old plastic dust bin for a water butt, the up turned lid collects water and when full it gets tipped in. An old plastic dust bin stores next years manure or can be used to let leaves rot down in. Flower buckets to grow veg in. I've cut the bottom of those too and use them in the GH as bottomless pots to grows stuff in the GH bed.
Wooden fruit crates are excellent for putting potted up plants in. The containers take-a-ways come in are good to freeze stuff grown in the garden and great to store various feeds like bone meal in or bird seed, slug pellets etc, and. they are mouse, nibble proof. I have a problem some years with mice in the shed nibbling through things.