You might examine each of the web sites of the eco-apps highlighted from this aggregated site: http://www.triplepundit.com/2014/04/3p-weekend-10-eco-apps-actually-worth-downloading/ or others to get inspiration (for number 5, Seafood Watch, use this URL: http://www.seafoodwatch.org/)
GiveO2
Leafully
GoodGuide
PaperKarma
Seafood Watch
Waze
Carma Carpooling
There are plenty of additional apps as well
If you want to design/implement an app be prepared to give a one-page specification of a mobile app in a sustainability area of your choice. One-to-two paragraphs should clearly lay out the intended tasks of the app; a third paragraph on how you would evaluate the utility (“success”) of the app. Optionally, include a picture (hand drawn is fine) of one or more interface screens of the app.