One thing the Midwest Academy excludes from their criteria is that the issue you choose is incredibly critical in determining who will be interested in joining your group. If you spend all your time working on forest activism, global warming, and recycling, then your organization is going to be extremely white and middle-class. If you work on community issues you may be working with more people of color and members of the working class. Here again you might want to try working both on a campus issue (which is probably easier to mobilize students around it) and a community one. If you care about diversity (and you should!), trying working with campus workers (support unions), workers off-campus (ex.: farm workers, city campaigns for a living wage), and tackling cases of environmental racism (and classism – where corporations are disproportionately dumping toxins on people of color and the poor).
You can also frame and organize around traditional environmental issues so that they become environmental justice causes. For instance instead of working to ‘save a forest’ you could work to ‘support indigenous rights’. Include an analysis of where your university’s waste goes in your campaign to create or improve your recycling program. It’s probably getting dumped on poor and people of color communities. Support the struggles that concern people of color, the poor, women, and queers.