I like the suggestion about doing yard work. DH would pay a lot more than $20 to have our driveway shoveled, leaves raked and cars washed!
Other odd jobs I did as a kid: pet-sitting, garden maintenance, farm work (picking fruit, veggies), hauling firewood, clearing brush from undeveloped lots, making holiday decorations.
You could also start garden seeds and sell the grown plants. You could combine the landscaping brush-clearing service with the holiday decorations--save the evergreen brush clippings and make them into holiday decorations. You'd still have to buy ribbon, candles, bells etc. for the holiday decorations, but you'd be getting paid twice over for your work. Similarly, you could combine pet-sitting (say, sweeping out chicken coops) with garden maintenance by bagging the composted manure for use as garden fertilizer.