I, quite often, have a seed or a cutting in my bag and, if I see an appropriate piece of soil on my route, I just plonk it in the ground, you don't even need gloves or a spade, I often just use a little stick to make a hole in the ground, fill it in and firm it up, and keep in mind that you planted it there so on your way back you can give it some water. And then at first, everytime you walk past there give it a bit of water and tend to as neccessary until it can become independant...! When you set it free and it lives it's own life. But everytime you walk past it you will still secretly fill with pride and love.
If there are no suitable beds in your area... (Sydney council likes to fill up their garden beds with this bright red gravel, which they've bucketed from somewhere out in the desert), then you have more work to do... In fact.. that is usually the sweatiest part of the job... Look at the seed bombing forum, these may be helpful for you