Mowing lawns for the neighbors (or your parents) is still a viable way to make some extra money during the warm months. And if you live in a snow-prone state, you can extend your services into winter with early-morning driveway shoveling or snow blowing.
Like pet sitting, lawn mowing offers an excellent opportunity to undercut the prices of professional competitors. The least expensive lawn service will charge a minimum of $30 for a small yard, not including things like fertilizing, weeding, shrub-trimming and crab-grass prevention. By knocking on doors, you are likely to find a neighbor or two who wants a cheaper alternative. You can decide whether you just want to focus on grass cutting or include other services like weeding. Because this is tough work, you might want to pair up with a buddy to save time, though you'll have to split the profits.
For safety reasons, you should be over the age of 12 to operate a walk-behind mower and over 16 to operate a riding mower [source: AAP]. The same caution should be used with snowblowers, so ask an adult before using any heavy machinery.