Egg carton as seed-starting greenhouse
If you are looking to start a vegetable or flower garden with your kids, the plastic carton that eggs come in with the anti-crush internal lids make great greenhouses for starting seeds. Just cut off the external lid and punch an air hole in the top of each dome of the internal lid, fill the bottoms (where the eggs were) with potting soil and plant your seeds.
I recommend 2 or 3 seeds per section; they can be thinned later. Leave the top down over the soil and place near a sunny window to make a nice warm greenhouse for the seedlings. You do need to water frequently as the little cups don't hold much soil.
I've also found that the clear plastic boxes that hold berries and other fruit work well, too -- and the holes are already punched in the top!
Related: Toilet roll seed starter