My fiance also wants to the donation as a favor. I plan on doing the OOT bags, fans at the ceremony, maraca at the reception (for use instead of tapping a glass and can serve as a semi-favor), and a card that says a donation is being made.
At present, I don't think we will do favors at the AHR (which will just be a party with hors d'oeuvres). We plan on getting koozies printed with our logo for the OOT bags but the orders are cheaper in bulk so we will likely have enough left over to give those away at the AHR.
I know that for local weddings I don't ever appreciate the favors and guests know that destination weddings don't have the same "rules".