For me it's things like:
There are always Tetrapacks left by the sink waiting to be rinsed out, or just taken out to the recycling box.
Similarly, plastic cartons and anything else that can't go in the dishwasher.
Piles of letters etc that I leave out to remind me to action them. Then I don't.
Loo roll innards left in the bathroom, again waiting to go out to the recycling. Hmm, there's a bit of a theme here!