Native landscaping looks nice, saves money, saves time, and supports wildlife. Win all around! The more we can do of this, the better.
Street trees give many advantages too. They are essential for a healthy environment. Planting trees promotes community.
Community gardens provide a place to grow produce and flowers. Here is a handbook for developing them.
Guerilla gardening uses space that the gardeners don't hold title to. (It's ridiculous to imagine that land can be owned; a popular delusion, but some people still work around it.) Seed bombs make it easy to plant hard-to-reach places. By far the best choice is using native species, because they require no coddling. I highly recommend that you get a bulk mix of natives suited to your area. Make a core of compost and soil, roll it in the seeds, and coat with a clay jacket.
Build new parks through incremental improvements. Think like Stone Soup Park. Parks offer a lot of benefits.
Unused parking lots can be put to more productive uses. I'll add another: convert them to a skatepark or to a playground with painted games. However, also consider depaving them.