As people are using AFK fishing farms lately, lots of trash is collected along with the fish, and wanted books. I recently built an own farm as well for that purpose, but rather then just throwing away the trash into a cactus or lava, I use it as fuel for my auto smelter.

Yes, fuel like (char)coal, Blaze rods and lava buckets are in huge numbers available, but using the trash you collect (bowls, worthless fishing rods, bows and sticks (even though I collect those for tools), you'll save fuel which could be used later on! According to the Minecraft Wiki, for example, if you use 6 trash bows from fishing, you'll save 1 coal block. This isn't much at the beginning, but the fuel you'll save up will stack over time, and that'll save you a lot!

This is just a small idea I got yesterday. It may seem stupid and the savings might be negligible, but it'll still save you some coal blocks in the long run.