My sustainable idea is Community Composting, a practice gaining popularity both in the United States and in Canada.
The idea is that a municipality or local group provides a compost pile that can be used by members of the community.
My preliminary idea ( which may change...they all have so far!) is support composting as a personal choice, then as a community to "grow a health community", and then grow a healthy world.  I haven't decided exactly how I want to illustrate it yet, but the layout of the e-card will be like this: