Have long term (3-6 year) projects broken down into semesters. As mentioned rooftop gardens is a great project. You could have students doing initial research, and development of all aspects of the project (architecture, finance, engineering, plant sci). Within the description/requirements of the research, there would be a meeting with other students working on the project and future project members. All of this would be for credit and would have concrete results.
Idea No. 48