Sharon, what a great idea! I've never attended or planned a "green" reunion and have not given it much thought until you mentioned it, but I do like the idea.
A few quick thoughts come to mind, some of which folks already do:
-smaller flyers (perhaps 4x6 and smaller instead of 8x11)
-love the canvas sign idea mentioned
2) Gift Bag
-something people can use during the year and perhaps an environmental educational component to encourage family to use them throughout the year
-re-gift or encourage re-gifting to someone who couldn't attend, perhaps, for those who do not tend to use their bags or t-shirts
-(would have to give this A LOT more thought because food is a big part of a reunion), but seems like there might be fun things to do here.
-I love the recycling idea mentioned
-perhaps having it at least one year based on the greatest number of who reside in a particular city/state. (energy efficiency).
-encouraging carpooling to reunion meetings/ the reunion itself.