Reusable centerpieces: potted plants which can be replanted or instead donate your cut florals to a local charity (i.e. seniors' home, hospital)
Organic and fair-trade florists
Reuse your linens as much as possible throughout the conference
Stick to cotton linens: refrain from using vinyl or other synthetic products
Low emission transportation: biodiesel busses, etc.
Green activities or community projects (i.e. shoreline cleanup)
Host a green speaker to inspire your delegates
Carbon offset programs for your meeting or corporate travel
Transit passes for delegates
Keep your organization and delegates informed of the environmentally sensitive decisions you're making for your meeting