You can’t get much greener than a push reel mower, and using one also allows you to get some cheap and easy (and productive) exercise. And they’re virtually maintenance-free, other than the cutting edge needing an occasional sharpening. There are some downsides: they don’t do a good job of cutting long grass, so you need to mow your lawn as soon as it needs it. Sticks will get caught in and jam the reel mechanism, so you’ll need to stop and remove them. And they don’t cut grass blades as cleanly as a motorized mower. But you can use one anywhere, at any time, and there’s no need to deal with gasoline, batteries, or extension cords.