Minecraft wiki entry for Farming, associated text: Cooked potatoes or chicken yield more food value, or compost, than raw.
Cooked kelp can be composted, or crafted into blocks for fuel.
Green dye (from cactus) has no use beyond making things green, so it may be necessary to automatically discard it. A clock-driven dropper can be used to throw the dye pieces onto a handy block of cactus, which destroys them.