DDs class must have done something similar recently, she came home singing "there is a worm at the bottom of the compost" I think there were also a couple of other mini beasts at the bottom of the compost.
I like the idea of sorting the pictures into the various bins.
For a game how about adapting "fruit salad" all the players sit in a circle, preferably on chairs. You choose about 4 things which could go in the compost, and go round the circle asssign each child one of them (eg apple core, banana peel and so on). Now take one of the chairs away so that one child is in the middle of the circle. They call out of of the fruits and all the children of that fruit have to run across the circle and swap places. If the child in the middle calls "compost" then everyone has to swap places. The child standing in the middle has to try and sit down. We play this a lot and usually find the person in the middle changes regularly but if the same person has had a couple of turns we'll ask them to change places with someone so everyone gets a go in the middle.