I read online somewhere that you can take the cut-off ends of your green onions and lettuce heads, stick them in water, and re-grow your produce. This is cool because the energy barrier between what you already do (buy these things) and growing them was basically nothing. You literally take the used-up ends you were going to throw in the compost, and you put them in a bowl of water.
Mine have been going for about a week now. Look how great this is!
Note on the lettuce: you can see that only one of them is growing. I guess you have to leave a couple of baby leaves on for this to work.