We are putting together very nice welcome bags that contain some things that might otherwise be given as favors. We are also having a dessert party the night before our VR that will be attended by all of our VR guests PLUS about 20-25 other people and we are giving out small favors there.
For our reception, we are not doing favors. Our VR weekend theme is "All of Our Wishes Have Come True". In lieu of favors we are doing a monetary gift to the Make A WISH Foundation. There will be a star at each place setting that will announce the donation in lieu of gift. Nearly all our guests are adults who will appreciate the idea of the donation in their honor, rather than a favor of some sort. I hope that helps!
Also, to the OP, if you are looking for a Marine-related organization, it might be really fabulous to make a donation to Toys for Tots (that's a Marine sponsored event, right?) I am sure they accept donations all year round. I'd LOVE to run with this idea -- for example, every place card could be in the shape of some kind of a toy -- a picture of a bike or a basketball or a doll with the person's name and table number and a note on the back about a cash donation to Toys for Tots!

I'd bet that would be the most memorable "favor" any of your guests would ever receive at any reception, and it's one they'll remember and talk about for years. And I'd bet word would get around about the charity, too! Good luck!