Nothing but editorials
One of the easiest ways to sell pictures of people doing things is to submit them as an editorial image because you don't need a model release when used in this context. The alternative is to arrange for a model and set up planned shots. Sometimes, this creates an image that does not feel as real as a candid shot taken of an knowing or unknowing subject doing something naturally. Unfortunately in the later case, most of the time you are not going to secure a model release from people who don't know you without them asking for financial compensation or just saying no. As seen in this image from Washington DC, it would have been impossible to secure model releases for everyone in the image.
To help get around this, we, as photographers, can take their picture and on stock photography sites, sell the picture for use only in an editorial setting.
While people make up one aspect of editorial images, copyrighted material, companies, trademarked and anything that could be considered illegal to use without sole permission from the company or individual, make up the other side. As seen in this image of vending machines in Japan, anything can be used as long as the composition is well done.
Like mentioned previously, we as photographers can use any of these subjects without their permission as long as it's approved for editorial use.
I have created a collection of a few photographers who have taken a number of editorial images and had success having them downloaded. Please visit this Editorial Collection to see more examples.
I hope you find what you need.
Photo credits: Tempestz.