I can't claim the idea was mine. Eggbox treasure hunts are a well used idea. But I can claim that, for an easy hour sipping tea while the kids rip flower heads off of your garden plants, this is it.
Take as many egg boxes as there are kids. Paint the egg holes different colours. Type a list of things to find. You can make out mine in a picture. Let them loose, with the instructions to avoid nettles, not rip the entire plant out, and, for the older ones, to get as close to the hue in the eggbox as possible. I left out a book with different types of wild flowers in it for the older ones, so they could label their finds afterwards. Kit yourself out with a jar for the snails, if you are crazy enough to put them on your list. For me this was a no brainer, as the sodding things are eating ALL my spinach, and I have the added bonus of introducing the concept of life (death) cycles to kids via getting them to fling the snack snails at the hens. Do not put the jar in the kitchen, as I did, and forget about it for 24 hours, to wake in the morning to silvery trails. Get one magnifying glass.
Send them out into the wild. Let them get the stuff, run riot, argue over what is orange and what is yellow. Then, because I am mean, make them draw a picture of a close up, using the magnifying glass, of a snail, or whatever more sensible object you get them to put in the jar, and write a sentence about it. Education over. Feed them with sandwiches and the Haribo as prizes.
The real genius of this is that it kept 9 kids utterly occupied for nigh on 2 hours, and we mums could have a chat.