I like a mega challenge that is manageable because everyone can do their tiny part. Like the Justdiggit project [0], on a mission to regreening Africa and cooling down the planet. All one needs is a shovel and knowledge how to dig shallow holes with a ridge to avoid rainwaters flushing furtile topsoil away, and store water for plants to grow in the dry season.

[0] https://justdiggit.org/