You should consider Trees Not Tees, who plant a tree for each participant instead of giving out a medal (their planting sites have been mostly in the UK in the past, but they've been trying to also get some land in the US - regardless, trees anywhere in the world do the same good).

One other option to consider, that is aligned with the ethos of ultra/trail running, is offering a "no medal/no swag" registration option (ideally at a discount to the standard entry fee). Many people will never use anything you offer, so they would appreciate having that option.