It depends on your state’s policies, actually; many states prohibit the donation of leftover food because its safety can’t be regulated (donations of labeled or packaged food is always okay, though). The best way to find out if you are able to donate your leftover wedding fare is to find your local food bank and ask.
If your state won’t allow donation, consider the possibility of providing chic “to go” bags for your guests to take home leftovers. Your caterer may even be willing to fill them for you!